Do you really think they can make LOHAN's mighty lifing apendages strong enough to endure 80g launch accelerations? I would hope not. (Probably 8g is already pushing it in terms of structural loads.) Yes, a piston launcher gives more force (like I admitted already) but A: it could be too much force, and B: you'll be putting a lot of that energy into kicking the girder+payload backwards, wasting a lot of it. (Even at 5 kilograms girder+payload weight, thats only a factor of 10 LOHAN to launcher ratio, assuming 500 grams weight for LOHAN, which might be a bit low)
A piston launcher also provides much more sliding surfaces to seize up, goes against the KISS principle and would be heavier than a launch rail/rod. (Every bit of mas you don't have to lug up to altitude means better climb speed and thus less drift, or more payload weight available for other useful stuff)
"Can we agree that this is basically the minimum possible speed we can expect aerostabilization — that it would be silly to expect a rocket-plane in 5% of an atmosphere to be stable at under 80 km/h?"
Nope, I'm afraid I think it wouldn't be silly to expect that. I've seen model planes weighing close to a kilogram being stably controlled at less than walking speed (maybe 2 or 3 km an hour) at our normal sea level atmosphere. With some proper design and some sufficiently large control surfaces 80 km/h COULD be sufficient for aerostabilization at 5% atmosphere.
Like I said, it might be best if we could get an answer on the matter from the LOHAN design boffins themselves. Afterall, they are receiving/have received the proper education and tools to simulate, predict and calculate all these factors.