Did you miss the part where Doc tells us it already has a working magazine? It's a under barrel tubular magazine much like most lever guns, and currently it requires a hex wrench to remove the end cap when reloading. The magazine spring Doc used in this prototype is a bit short so he made either an elongated follower or a spacer put behind the spring, don't remember which, and in this configuration the magazine now takes ten paint balls. With a longer spring and no spacer he estimates that the capacity should be at least 15 balls.
One of the early posts in this thread asked about a side port for loading the magazine. Doc answered that he'd considered it but hadn't been able to figure out a way to do it, at least not without putting enough pressure in the balls that they risked to be broken during loading.
Suggested solutions included milling a slit in the magazine tube, allowing the follower to be manually retracted during loading to take the pressure off the balls.
Personally I think the easy solution might be to mill a L shaped slot in the end of the magazine tube and make an end cap with a pin that will fit into that slot. So to load the gun you tip a tube with 15 paint balls down the magazine, insert the follower - spring - end cap assembly and twist the end cap so the pin catches in the slot. To make this work the spring pressure when the magazine is empty has to be strong enough that the end cap won't spontaneously unlock. Another possible problem is to keep the spring from buckling outside the magazine tube as it's put under pressure. It might me possible to use a guide rod attached to the end cap, or even a telescoping guide rod, one part attached to the end cap and the other to the back of the follower, with one of them being slightly smaller and telescoping inside the other when the spring is fully compressed.
I might be overthinking this. If it's easy to compress the spring without having it buckle then a guide rod would just be an added expense.
The follower, spring and end cap will probably have to be connected in some way, or loading might turn into a two person job.
The front loaded tube magazine was actually used by at least one lever action rifle, the name escapes me though. I also remember something about another rifle with a slot running the length of the magazine tube, but I can't remember if it was for retracting the follower, but I can't really think of any other reason for it. I do think I remember that the slit weakened the magazine tube so it was easily damaged.
But this being Doc's love child I'm sure he will come up with some slick solution.