Even if they're in the same HQ unit, the odds of targeting the supply units is fairly slim. I assume they're marked as non-combatant, which lowers the chance of targeting. For the support fire to target the HQ unit, one of your other units has to target them, which assumes either their HQ isn't in rear echelon or you've already killed most of their other units (I don't recall off the top of my head but I'm fairly certain that in a breakthrough the support unit doesn't get to fire again), and if the element chosen in the supporting fire is artillery then ALL of the supporting fire for that attack targets that artillery element (counter-battery). In theory, yes you could kill supply units with LR and HB, but the chances are slim.
We can estimate the odds for a first approximation:
- Assume that the average size of an NPR ground formation is roughly constant and that NPRs use a 3:1 ratio of HQ to line formations. If the HQ formation is supporting one of its three frontline formations, there is roughly a ~5% chance that it will be targeted directly in the support echelon (which is where it must be to lend MB support), and about a ~30% chance that the frontline formation it is supporting will be targeted.
- If the formation supported by the HQ/artillery formation is targeted by a supported player race (or other opponent's) frontline formation, there would be about a 20% chance for the HQ formation to be targeted by counterbattery fire. This gives a total ~12% chance for a HQ formation to be targeted by your own supporting artillery.
- Going from the numbers in Andrew's post for an example, 200x LVH+LOG = 12,400 tons, if these are non-combat units as they should be their effective target size is 3,100 tons. Assuming the HQs are also noncombat STA with an effective target size of 131 tons, and the CON units have an effective size 159 tons - the LVH element has approximately a 30% chance to be selected as the target.
- Taken together, this gives about a 3% to 4% chance that our artillery will target the LOG element. If the NPR uses MBL or HB these odds probably drop below 1% due to the fact that rear echelon formations are even harder to target as their effective target size is 4x or 5x lower than even a support-echelon formation.
Basically, we can draw a couple of conclusions. First, there's not really a way to effectively target enemy logistics units, in the best case we simply get lucky that the enemy has mixed supply and bombardment elements in the same formation. Second, it is not going to be an effective way to attrit enemy logistics. Estimating a very generous ~10% hit rate and three shots per round, it will take about 100 rounds of ground combat to destroy a single LVH+LOG unit per artillery gun. This is over a month's worth of combat by which point the battle should already be decided, and to be frank by this point if you are still fighting the NPR they are probably already out of supplies anyways. Point being, you'll do better to just straight-up kill the enemy troops which you are pretty much forced to do anyways since, per the first conclusion, you can't really target the enemy logistics units very selectively anyways.