Edited my above post a bit but thanks for the info
FYI the project is adapting a PS2 beatmania IIDXcontroller to USB, it's a music game which has a turntable. The turntable uses a quatrature encoder, and some chip in the controller takes the sensor output and determines direction from it, then outputs left or right to the PS2. Unfortunately that chip that does that is also the main "talk to PS2" chip under a glop-top. So if I were to interface it to USB I would need to do the quadrature sensors -> direction handling on my own.
There's homebrew beatmania IIDX recrations on PC and the old "beatmania" (non-IIDX) series is emulated well in MAME, but using a PS2->USB converter introduces lag, I would rather natively interface it to USB by wiring the controller's buttons and turntable to a USB gamepad.