OK the way I understand it is when you fill a 20oz p-ball tank you do so from a siphon tube equipped larger tank. This is to transfer the liquid and you only fill to around 80 percent to prevent a burst disk malfunction. When you use a tank for beer, you are using a non-siphon tube tank, and only getting the pressurised gas that "boils" off the liquid inside, the same as a paintball tank does. That's another reason they have that expansion chamber inline. If they didn't, you would shoot liquid through the gun, destroying o-rings, blowing up paintballs in the barrel, and freezing the gun's action after a few shots. That's why you have to run through so many of those little "gram" tubes, they hold only gas, with no liquid to boil off. Not that I'm an expert but I have 5 boys who play constantly. Anyhow, I would think that a 20 oz tank through a single gauge regulator, and hooked to the keg would work. As long as the tank is kept upright. You'd be basically doing the same thing as hooking up a 20 lb tank for the beer anyhow. I'd like to see how you do this, if you decide to.