And the last time that was tested with Connectix and Sony, Connectix won in the end but faced so many legal fees they still had to end up selling it to Sony and abandoning it.
Sure, and companies could also sue you for libel when you leave a negative product review. Life is unfair, but when talking strictly about what's legal or not, homebrew roms using an emulator that has a clean room implementations of BIOS should be perfectly acceptable for the App store legal-wise.