XGS- Apple II GS Emulator
From what I've gathered, this is coded in C . Possibly it could run at reasonable speed on the DC (a Pentium 233 should be enough to run it full speed on a PC if I remember the readme right).
Source is available here:
http://xgs32.emucamp.com/, (edit: Just realized, this is already a port - for Windows, I think the original emulator is KEGS, which someone ported to sdl apparently here http://devernay.free.fr/apple2/)
I think it has been ported at least to the Xbox.
The actual usefulness might be rather limited of course, most games are also available on other 16bit computers (some exclusives exist however). From what I've gathered the GS is however backwards compatible to the normal Apple II models, so that would be a huge plus, since the Apple II emulator on the DC isn't quite advanced and not that user-friendly.
Anyway, it's just an idea! Mac emulation won't happen anytime soon I guess.