Indiana Jones and the Infernal Machine was made by the same people, and there was a cheat code - "MAKEMEAPIRATE" - that turned Indy into Guybrush Threepwood. Best of all, when you used the bazooka in this mode, instead of a rocket it would fire a rubber chicken, which would swim through the air at a bit slower than Indy's running speed, emiting the Monkey Island music as it went, until it hit something and exploded.
Also, if a cutscene happened during this, you would look like Indy again, but wearing Guybrush's clothing.
Also, in one of the last levels, there was a secret tunnel you could go through that would take you to a replica of some sort of cabin from the games, maybe Guybrush's home, and turn you into Threepwood while you're there. There was a treasure chest you could get and everything.