So I ultimately gave up after a while, but some recent developments had me figuring out a weird work around!
If you have the Playstation Riffmaster, hold down all 5 frets and then plug it into your PC with the USB-C cable. The PC will recognize it as a Xbox 360 controller, and it will naturally be found by Nyarupad and respond to the frets! The downside is you lose tilt and whammy, but that was already an issue with the PS riffmaster on PC. The stick also stops working. It also means Fortnite Festival will work without needing the Festival Instrument Mapper. And Clone Hero/YARG/GHWTDE should recognize it with no issues.
The only other option I can find with Nyarupad and Guitar controllers is to just find a native Xbox 360 Guitar. Other console guitars, adapters, and Santroller guitars just doesn't work with Nyarupad unfortuantely.
Hope this helps!