well, the rom is a direct port, so to speak, of the game, so if you could train pokemon on the n64 version without a game boy gamepak, then you can do it there. Check gamefaqs.com to see reviews and stuff like that.
Also, maybe you could get a gameboy emu, play it, save it, and then feed the data into the n64 rom. Would that be possible? Maybe someone with a lot of emulator experience could say.
-Jimbot