Well, sorta. However, you don't have to wait for marshalls cart emulator. You can make custom variants of Super Mario 64 and play them on Project 64. Just google Toad's Tool 64.
He said programming his OWN game, not rom hacking. Even though TT64 is pretty awesome, its not what he asked for. (although sadly, like Lunar Magic, it has caused a flood of just horrible rom hacks, with few gems in between)
