The only way Brütal Legend could help make a Psychonauts sequel is if it becomes a wildly out of control run-away hit. Then a sequel will be rushed into production while quick knock-off sequels will be made of anything Tim Schafer ever touched, a la American McGee's Alice. It would be a purely money grubbing effort.
Otherwise any success Brütal Legend has will just fuel calls for its own sequel.
If you've listened to Tim Schafer refer to the Psychonauts character development process in his talk on character design, you'll realize that it probably can't be duplicated for a sequel.