The massed launch from the extreme limit would give the defender a great deal of time/opportunity to plot interceptions and organize a defense.  But it would make destroying the bug ships very difficult - if not impossible.  But from the fiction I have read, it seems that building big and fast missiles is expensive and time comsuming.

And if you can make 'extreme velocity missiles', they will only get faster if you ramp up the launch vehicle's speed before releasing the missile.

Missiles are pretty much by definition going to be cheaper than similar speed disposable ships. Even if you want to accelerate the ships before launching, just do that outside the hyper limit; since missiles are essentially infinite ranged you don't gain anything by sending disposable ships in then launching.

An fighter or missile heavy strategy would be a good counter to this though.  Your strikes launched at long range could be intercepted by small mobile elements, AMMs and pointdef FACs, so the humans might not be drawn into a combat towards the outsystem. 
Even if they need to move ships around, intercepting the shots can be done from where they currently are. 

Especially if you only jump in with a fraction of your force, if the commander knows what he's doing, he will not commit overly to chasing light skirmishing forces with the majority of his defences.  Especially when it looks likely they will bug out before anything can reach them. 

Multiple waves won't do anything to change it.  If I decided that 20% of my forces is enough to take your skirmishers on if they hang around, when the 1st lot jump out and the 2nd lot comes in, that same 20% will be tasked to take on the new guys. 

EDIT:
It strikes me that this is a good thing I just learnt. 

Forces can be committed solely as a function of their current movement vector, not their current position. 
Do not over commit forces against inferior enemies in case he's drawing you into a trap or out of position.  Use as much as you require to win, but have reserves, especially if you are required to defend an important target. 

Although I did specify that if clearing the colony was required to kill the human forces, then the Bugs consider this an acceptable loss. 

Depending on diplomatic intelligence, if the Bugs know that the humans will defend their population as top priority, then it might make a perverse kind of sense to send a wave of missiles against the planet.  Stoppable missiles that will force the humans to intercept them so their ships can be defeated in detail.  =D

Workable?  Maybe. 

It has been fairly active here at work so I haven't had much time to formulate on strategies.
The defense scenarios look well thought out.  It may have a few weaknesses, but it may be hard to analyse without actual game time to find out.

For the attack plan, I may make a few changes.  This game is going to be based (to me at least) on something close to the principle of chess.  The winner will be the one to make the next to last mistake.  The trick will be getting the other side commit the telling miscalculation.

To this end - only parasite carriers will be employed.  All tonnage dedicated to battlecruisers/jump drives of support ships will be used to build additional parasites in origin system.

No support vessels will be jump capable.  All will remain in 'origin system'.  All are to be distributed as best as possible to intercept returning carriers to refuel/rearm them in the origin system.  All support operations are to occur in origin system.  Only combat will occur in target system.

I would jump in only a few (probably two or three) jump ships to begin with.  All parasite carriers.  Parasites will mount two 1kg railguns of smallest size, plus single laser mount of minimal size and all need to add at least four AAMs/two shrapnel and two nuclear.  Aim for the maximum possible jump distance from the primary.  Attempt to set initial heading for at least 60 degrees off axis from the primary.  Attempt for even distribution around primary.  Set course as close to possible so that one ship will be moving towards the insertion location of the ship immediately clockwise of it (although counterclockwise would be fine).
The goal here is to have each ship land and be pointed as closely as possible at the landing of the ship immediately clockwise of it.
Jump in will be at maximum possible velocity achievable with 30% of onboard fuel.  Ship will proceed on this course until inbound launch is detected, firing of weapons is detected, or 48 hours have passed.  At this point all ships will make maximum delta v burn at 90 degrees from primary to accelerate to outer system and attempting to create smallest closing speed from any incoming weapons.
Jump prep will begin immediately upon arrival in system.

After outward burn (up to 20% of fuel), half of parasites will be released.  Additional 5% of fuel will be burned to be followed by release of all remaining parasites.  Following each launch the carrier will attempt one degree course alteration preceding next launch to ensure parasite dispertion.  Parasites will be passive release to minimize sensor signature. This will result in two 'lines' of parasites trailing the carrier.

Any and all launches/firing will be recorded by carriers, as will inbound courses of any ordinance.  Carriers will maintain course until any parasite or ship detects incoming ordinance with less than one minute to TOT.  Jump for return to origin system will occur at that point.  Remaining fuel will be used to facilitate rendevous with support vessels in origin system.  New parasites will then be placed on carrier as available.

All 'first launch' parasites will begin burn of no less than 50% of available fuel toward location of any noted PDC or station launches/firings.  At completion of burn all parasites will launch two AAMs (one nuke to be followed by one shrapnel) at any stations or PDC on airless targets.  Missiles will use 5% of fuel to displace current course from target and them burn 80% toward target.  Remaining 15% will be used to ensure intercept of target/evasion of incoming ordinance.  Nuke will precede sharnel missile by no less than 100 km.  Nuke will be used to intercept any incoming missiles/ordinace.  47% of all expendable rail gun ammo will then be expended on same target following AAM launch.  Parasites will them procede to complete burn of 25% remaining fuel to attempt intercept of nearest 'rally point', which will be the location (or as close as possible) to the first follow on group's insertion point.  At this point remaining missiles will be launched on second identified stationary target.  Missiles will follow same displacement/burn as initial launch.  Attempt to coordinate TOT if possible as priority.  Following launch all but 4% of railgun ammo will be fired at second target.  Remaining railgun ammo and laser installation will be used to deal with incoming ordinance.  Remaining fuel will also be used for evasive manuevers and 'landing' with follow on carrier at 'rally point.'

48 hours after initial parasite 'line' expenditure of all ordinance, second line will begin burn using same tactics but reserving 20% of initial railgun load, with priority of fire to #1 - achieve TOT on new vector to previous target / #2 - fire mission on any stationary installation noted as firing on first line parasites or ordinance.  Second line will complete only single launch on stationary target if any mobile targets are identified.  Second line will then complete burn toward any mobile target noted so that no less than three parasites will engage the mobile target from as dirvergent of vectors / coordinates as possible.  Remaining railgun ammo/ AAMs/ and laser will be used at pilot/group leader discretion so as to best bracket mobile targets for destruction.  If no mobile targets are identified second firing using remaining AAMs and 6% of railgun ordinance will be expended according to prior priority listings.

All surviving parasites will attempt to coordinate landings with follow on carriers at predesignated 'rally points.'  Second group of carriers will arrive and follow prior tactics for launch/manuever while attempting to land any surviving parasites of initial wave as possible.  Priority of fire will continue for stationary targets with mobile targets being engaged as possible.

The second and third follow on waves will attempt to identify target installations and ensure maximum expenditure of target system munitions in attempt to intercept our vessels/ordinance.  Priority is also to draw all mobile assets towards our vessels/parasites to ensure vectors away from target population.  When mobile defenses have been committed (at judgement of commanding intelligence in target system) and are judged incapable of further manuevers to intercept any ordinance fired at target population - third wave will broadcast demands for surrender.  If refused - fourth (or fifth) wave will be launched at target population.

This wave (to include all available carriers/parasites) will be placed at closest possible jump distance from target population at max velocity acheivable with 60% of fuel.  All additional fuel other than necessary for jump will be expended toward target population with random 0.1 degree course alterations.  All parasites will launch following full burn or as soon as possible before any detected inbound ordinance intercepts.  Parasites will also complete 100% burn following same evasion protocol.  Following full burn or prior to identified intercepts by incoming ordinance parasites will launch all AAM to ensure as best as possible coordination of TOT for all carrier groups of munitions on target.  All AAMs will be nuclear warheads of no less than 1 MT if possible.  Following launch and displacement manuever of missiles rail gun and lasers will be employed to attempt to intercept/destroy any defenses of population that would be capable of missile intercept.

Carriers and parasites may be considered expendable in final attack if necessary for destruction of target population as necessary.  Any not destroyed will continue past target until jump possible and them attempt return to origin system for rendevous/resupply.

EDIT

This is just a quick first go at it and will be subject to major revision once I see what you can actually accomplish in the game....
 

The goal is to get the defender to commit most of his defenses to outsystem vectors, and then jump in and create a wave of death approaching the target population at a speed minimizing target intercept opportunities/time, and that they will be unable to intercept with remaining defenses due to them being committed to vectors that will make it impossible.

It also considers most of the attacking parasites as expendable by nature.

EDIT II

On additional thought - in that the final wave's goal is just population destruction if you are able to draw the defenses out/destroy them - it could be possible to use purpose built cruisers designed simply to punch through the remaining defenses at high speeds and shower the target with a mass of AGMs/AAMs.  So long as they show up as close as possible to the target, have a high acceleration, and enough defenses to unload their weapons before they are destroyed, the missile cruisers could do the job.  They could also mount larger warhead AGMs (SGMs??) than the parasites.
Perhaps if I use strictly carriers they should also have the capability to carry a few 'planet killer' nukes for the final wave...  

So, as a side question:
Would it be feasible to just station a few dozen railgun armed pdcs on asteroids all over the system, with just a very small defensive fleet?

And as an attacker, if there's a lot of time, how about probing jumps at low speed, maul a few harvesters or other potentially stationary targets, then leave again?

I intentionally left myself unspoiled on the defenders tactics till I was done, and I did note that both have tactics in place to deal with deep space support ships. I suppose a lot depends on how easy they are to find (there's a lot of room for hide and seek in deep space) which might result in an interesting two-front battle.

For the sake of anyone who tries to predict two of them crashing, if the support ships got taken out I'd say the reduction in fuel would mean the bug force would probably have to go with much smaller velocities for the rest of the battle. And of course they'd only have whatever missiles were in their magazines.

While we certainly can be more realistic, I would like to keep this limited to what might be acheived in the actual NA game.  I don't think we're about to get sensor degradation in the presence of planets.  And slingshot maneuvers only work well if your speed is around the same as orbital speed of the planet you are slingshoting around, NA delta-vees are far too high. 

I suppose we can assume the main planet does *not* have a moon and both gas giants have many moons.