so, there's no way to change just one icon? I have Project M and it had the Dracula's Castle already in it over Luigi's Mansion. The logo was changed for only that stage. I think it's possible.
It looks like they added a completely new logo to the existing ones and likely used code to make Luigi's Mansion call that one instead of the mushroom logo. Hard to say what they actually did to make that happen without digging further into it, though.