Its not a very viable solution for E-PPG for many reasons. First, even if it didn’t make the battery pack any heavier it could save at best around one pound from removing about 3 feet of aluminum structural tube from the frame which weighs about 1/3 pound(if that) and whatever system is used to hold the battery on the frame. The reality is that it would almost be certain to make the unit even heavier because of the needed epoxy or other binder which would fill between the cells and would add a few pounds to the battery. The other reason is that at this point in time the best electric paramotors will get about an hour flight time which is why every unit on the market is designed so that the battery can be removed and replaced with a full one in a few minutes so you can get back in the air quickly.
The problems with fitting cells into the arms is that people will often break or bend those components and bad things could happen about the time some of the cells was shorted out, bent, punctured, or damaged in a crash. This goes back to why Tesla puts their batteries all together in central location of their car and encases them in a sealed titanium box. It would also make wiring the system nearly impossible.