Functional rear diffusers aren't going to be simple no matter what. It will have to be at least a few feet long on the underside of the car, and it will have to close off its own front edge, so yeah there's going to be compatibility issues with just about any aftermarket exhaust. If anything the factory rear exhaust section is probably one of the simpler ones to fit a diffuser over.
I wonder how hard it would be for someone to fabricate a complete undercover for our car? Replace the stock front undercover and make a relatively smooth underside all the way to the rear. At least then you don't have to close up the rear diffuser in the middle of the car (over the exhaust and driveshaft area). A universal full-length undercover with some diffuser fins in the rear could be custom cut for various aftermarket exhausts to fit through it.
Probably be hideously expensive and not sell many units though
