I spent a while messing about collecting co2 from fermentations in "expandable containers" and then connecting them to the beer, so serving sucked in co2 instead of air. Things like kids beach balls provide your saved co2 with no pressure, while large rubber balloons provide a tiny pressure. It worked fine, but looked pants. I ended up with what we call "polypins" as co2 collectors and that gave me 25 litre inflatable/deflatable containers, self contained in cardboard boxes. Those massive weather ballons always looked "usesable" during my mader moments, but I could never get hold of one.
Is there any reason why you could not connect a normal co2 tank via a demand valve like scuba divers use? Then you would get co2 supplied at minimal pressures as and when it is required? Just a thought.
Has anyone got actual pressure figures for casks in cellars in the average pub? What are they allowed to self pressurize up too when the hard spile is fitted between serving sessions and also when they arrive from the brewery and are settling out. During serving sessions, the soft spile is fitted and so the pressure is zero and the idea is that the air enters the cask slowly and the air sits on top of the blanket of heavier co2 that is on the beers surface.