I could see it being useful for guitar players who need to be able to feel the strings, but at the same time I doubt the sensor would be able to see your fingers. At least when I play, my wrist is directly behind the fretboard. This sensor wouldn't see my fingers at all. It could see my pick hand though.
Acrylic neck and fretboard with a really thin truss rod? It would probably feel terrible, and the IR light might be distorted anyway.