I asked something like this two month ago in the PJ64 forum, and it seems it is useless as soon as the n64 is not perfectly emulated. An emulator is the best way for make changes quickly in the graphics or in the core, with a plugin system, better than recompile youre game every two days.
BUT, if someone interest to this, a GameBoy (or maybe GBA) translator would be possible. the code would be:
while(index < rom.lenght)
{ readOpCode;
translate OpCode;
}
run recompiledRom;
This system has been done with other systems, I don't remeber wich. And you can ever for the GB translate the GB-asm code to a GB-C code (it would be easier) and then compile the C code with GBDK.
You can do exacly the same thing with a N64 - theorically. The N64 is very complex, and only Corn was is his first versions a recompiler - after not. And remeber that translate a game from PSX to PC, for example, take 3 month to the originals programmer, with the source code. So with only an incomplete asm code...