I'd be happy to contribute my NES sound emulator to such a project. Much easier to just port the game yourself, as has been done with the few NES-to-SNES ROMs that are floating around. Wouldn't work for later games that used mappers, one would have to basically 'unroll' the mapper and generate a flat memory map that the tool could use to convert the game. Then an automated tool could take that external information and build an SNES ROM. Still wouldn't be 100%, so the user would have to manually ensure that all code was marked as code, graphic tiles marked as graphics, and sound driver marked as sound. This would require the user to play the game under such emulation and thorooughly exhaust every branch of game code that they could possibly do. One could make build the necessary SNES-side code and build a tool that would convert everything necessary - it would probably either require ROM detection (to know where data and code are in the ROM) or include a built-in emulator that logged every address that was code. It's possible to be semi-automated for a certain subclass of NES games - for example, CNROM games only.