You probably could just melt a hole in the top of a length of FBR then drop a few weights in the hole and fill with the PLASTI-DIP instead of hotglue.
These should work in modded guns fine if the PLASTI-DIP doesn't go past the edge of the FBR.
I have a pair and I'm 18, they are pretty fun to mess around on but the best thing is, if you take the wheel out they are pretty damn comfortable.
I payed roughly $65US for mine and I still think they were worth it.
The look on peoples faces is worth at least $50 some of the times.
Or you could have a catch on the plunger that when the plunger is 3/4 of the way cocked it catches the bolt mech and slides it back to load another dart and when fired the bolt will slide forwards with the plunger before too much air comes through.
I will go down for a kit if you start producing them, also the handle serves no actual operational purpose besides to hold the gun right?
If so if I get one I will probably sand and reform the grip a bit.
With the clip loading idea. It couldn't be that hard but would probably reduce range, all you would have to do is put the plunger above the barrel and then have vinyl tubing going from the end of the plunger tube to the back of the barrel, I realise that I have just given a very basic idea and it would be alot more work than that, but with what you have done in the past with PVC I'm sure you could work it out.