is it not possible to remove the ball stopper and then put in aftermarket detents (rubberized "ball stoppers") on the top side of the chamber?
The main problems I'm seeing right now is that RAP4 magazines shoot a smaller caliber paintball than the usual .68 that most markers use but I took apart an ar15 mag just to test this out, regular balls will fit in it, and its a good fit, just the right diameter.
But, the other thing is I don't think that the balls fill the entire magazine like in your design.
I was studying the picture you provided and to me it seems like that thing would jam like crazy. There is no real good funneling system to push the balls into the chamber. I think on the RAP4 mags there is a single tube running through the magazine that has a spring in it, pushing the balls upward through the tube, rather than the entire magazine being full of balls.