A laser tachometer works by bouncing a beam off something reflective.
On that note, a positional sensor could be made that uses similar laser tech. By making the reflective surface coded, you can read its position; like Morse code. So having two strips of reflector real close together, and then a measured distance, followed by two more strips. When the laser sees that, it can tell where the belt is. You should be able to get 3-4 positions on a belt that way.
Like: belt --- reflector [