I just thought, if one owned the Maya program, and used it to make a level, then used Psychonauts Explorer to check how Double Fine used Lua to script, and then scripted their level in Lua, and then got a version of Psychonauts Explorer that lets you replace in game files, you COULD do it, I guess.
If Psychonauts Explorer could edit the packages instead of just let you look at them, modding would be possible. You could change all of Raz's textures and stuff, and could completely change the game graphics. If Mr Gorman is reading, PLEASE attempt addition of this feature.
Maybe IF there is a Psychonauts sequel it could be modded.