Noop, that extra chamber on the Gas cylinder gun is for the action. The air source (compressor) just hooks straight into the firing tank.
Never mind the thing about loosing energy.
I think it would be a waste to discuss it further. I'm just set on firing straight from the main reservoir w/o a secondary chamber. 
I and my friend have both gotten multiple (albeit poor) shots out of our 3/4" paintball pneumatics. (His has a 2" tee piston, mine's 1.25"). Our respective chambers are size for our barrels, so we don't have too much spare capacity. Also, while my piston did have a spring as a bumper, his had none, and just rebounded off the air in the pilot volume. He uses a 1/2" brass ball valve, I use a blowgun.
My thinking is, if I can make the piston on my big gun reset without even meaning to, I can make it reset much quicker when I use a more controlled valve, smaller pilot volume, a bigger piston/barrel area ratio, and a bigger spring.
Heck, Hotwired said in this thread that he gets 5 shots out of his copperhead gun. http://www.spudfiles.com/forums/viewtop ... 94&start=0&
Edit: Hmm, looks like in the spudtech archives FMJ and SPG were discussing a concept similar to yours, Hawkeye. I can't seem to find it again, darn it. But he was talking about the CO2 feed coming through a schrader into the pilot area.