I know that MiSTer is already accepted as an emulation platform, but similar to the SuperNT being FPGA (but unofficial), the MiSTer (also FPGA and unofficial) has now been listed as a selectable platform within sr.c.

Would it be possible to have it as an option? Or is it not used widely enough in the community to warrant that?

Yeah, it's not a very popular platform at the moment, so ideally we'd like it to reach a certain threshold of interest/runners to display it like for SuperNT. With current SRC implementations, we're pretty limited on how we'd like to show supported platforms/emulators in general, hoping we can get a better solution for things like this soon enough. Thanks for bringing it up :)