I've always thought that 3D platformer camera problems would be solved by always having the camera at a set distance. The only time the camera causes problems is when it gets stuck on something or decides to zoom in because there's a wall there.
Why not simply make it so any geometry between the player and the camera is drawn at 15% opacity? Problem solved - now the camera can go anywhere without obscuring the player or his view.
PS. Psychonauts ruled. Best. Platformer. Ever. Despite its flaws.