@gkm not entirely sure this is exactly what you need, but in the following topic they talk a lot about analog input devices.
https://github.com/libretro/mame2003-plus-libretro/issues/399#issuecomment-457024703
It might be just a configuration thing... contacting the people there might give you insights as I really belive I've read spinners do work with mame2003plus.
See:
https://docs.libretro.com/library/mame2003_plus/#mouse-and-trackball-support