it is fully hollow, u knock on it- it is fully hollow, so I would think access is pretty easy just by removing the pickups. however, there is nowhere to "mount" anything. everything existing is just routed into the top. I'm not cutting holes in the top of this one (or back, or sides).
it sounds great as is, honestly, especially the neck pickup. still, it is worthy/i want...some sort of west coast/USA guts inside that thing. I may swap a pair of Novaks in at some point, which will be totally unnecessary, yet a definite upgrade. if i got a pair that would give me his Bisonic and his Dark Star in both neck and bridge (and middle), plus the mini switch that comes stock on the bass which is either a low signal cut or something phasey...it already has so many different sounds, i suppose that would be more than enough. i would definitely consider Alembics first though, that thing is worthy, and I have other/vintage Bionics. i need to break that bass out soon, definitely in 2018. it is #25 of 25 GSR, i got it from a NH employee after the factory closed, with no case or strap or anything. had to mod an Epiphone Cassady case for it- absolutely nothing else fit.