Originally Posted by Bobby_M
You want a gravity feed keg that maintains carbonation? It's not physically possible. You need to maintain a fixed pressure in the headspace to keep dissolved co2 from coming out of solution.
I sometimes brew root beer/ginger ale using champagne yeast and put it in a cornelius keg and I don't even need CO2 to get it out. Just the pressure gets it out.
So my guess is that if I would have a spigot at the bottom of a similar keg/jar, the air would get in every time the co2 pressure would fall to a certain level and then the top of the keg/jar would get new air to continue the fermentation.