The IIGS uses a 65816, the same 16-bit CPU that's used in the SNES. Since the MiSTer already has excellent SNES support, I don't think there's anything to be done as far as modeling the CPU for the IIGS.

This is a "fantasy" 32-bit extension of the 65816 that WDC specced out but never built. It wouldn't help with getting the IIGS on MiSTer.