There is another option. Put the bios inside the rom of the machine, make sure the rom is the one for external use. I think the RSBC has such bios that have both, internal and external.
I am curious to see the final project. I might be useful for me because I want to build sound cartridge for personal experience. I try to learn as much as possible.
I'm not sure it will work. A ram segment will be detected as ram and used like that by the running program, for instance to store data that will overwrite Fm bios code.
