yeah, that's what I'm thinking of. Although I can't understand how they make a seal to the cap, and I would like something I can leave connected without holding the chuck in place.
What I'm picturing has to be a two-part, threaded fitting, with a gasket on the inner side of the bottle cap. Then when you thread the two pieces together, the gasket is pulled tight to make a seal against the cap, and the end projecting out fits a QD from a CO2 line.
Could I adapt a keg post to this application? Or do they need the threads in the keg lid to work?