Oh damn, I did a test run on this, captured ship, but now I can't figure out how to get the Marines back off it.  Is there some way to transfer between ships?

You want to damage a ship you intend to board to reduce their speed.  Your drop capsule troops suffer loss based on the difference in speed between your target and the ships with the drop capsule.  To have no loss from the drop you need to be going 20 times faster than they are.  For example I have some FAC's with drop capsules that go 10,000km/s  If my target is going 5000km/s then only about 25% of my troops will survive the landing.  If instead the target was slowed to 1000km/s I would have 50% and if they were going 500km/s then most of my troops would survive the landing.  As you can see slowing down a target can be very important. 

Brian

I think I could whip up a 20k speed gunship pretty well.  Problem would be damaging the ship quickly; maybe some meson cannons on the assault craft?

Yeah, I was planning to have the assault ship have all the sensors and the dropship just be a really fast (~20k km/s) FAC for the actual drop.

I read that officers aboard NPR ships that have been captured will sometimes divulge information on their race's ship classes or a tech.  If NPRs assign officers to commercial ships, wouldn't it be possible to raid said ships with FAC-delivered Marines to gain intelligence? Pretty much it would go like this:

1.  Sneak a stealthy (e. g.  low signature) assault ship holding Marine companies and a rapid shuttle into a developed enemy system. 
2.  Use very large passive sensors on the assault ship to detect enemy shipping.
3.  After observing which planets ships seem to go between, set up on one of these shipping lanes and wait for a freighter to approach.
4.  Once a freighter has approached, use the drop-ship to initiate a boarding attempts at high speeds.
5.  Once the Marines have (hopefully) captured the ship from the civilian crew, you should get intelligence (if the dice rolls well, of course). 
6.  Transfer Marines back to assault ship, do whatever with the freighter (destroy it, stash it away, etc. )
7.  Run away fairly fast as the enemy race probably would have realized you are there by this time.
8.  Wait an appropriate amount of time and return.

Of course, speed would be extremely important or you would probably be blown out of the sky.  I haven't tested this myself but I may be in a position where this would be highly effective.  Can you see any glaring mistakes?