Ok, so I'm no machinist or engineer, but here is an idea.
Ever thought about a removable bottom feed tube to act as a replaceable magazine?
I had this idea about a flat spring loaded lock holding the removable tube mag "clicked" in place (into a "socket"), and at the same time this flat spring operating a sort of a ball detent keeping the balls from rolling out when the tube is not inserted. Once it is in place, it feeds.
Loading individual tubes could still be done from the other end, in case the pressure would be too much on the paintballs to be loaded from the "detent" end.
Would this be too flimsy? I don't know, but it might spark another idea.
I didn't consider how the operating "fin" could be fitted through/into the foregrip, but it is just a concept, a food for thought.
Maybe this single part of the rifle could be made of plastic (at least the tube) if we are talking about reloading in the field, as it would probably be dropped when empty and later collected.