I've actually thought of this. It could probably be made one stud wide with SMT parts and just plug into a sensor port. Or, make an adapter of sorts that lets you plug both the PF motor and the sensor into one motor port and use the existing tacho hardware and software in the NXT to count RPM and such. It would be hard to get precise movement, the best you could do would probably be around ~1 full rotation, and that's wishing
but it'd be great for counting precise RPM and detecting stalls.