i was in home browsing, then something appeared in my ****ed mind
try this
get an optical mouse, put it in top of an guitar/bass in the neck pickup position, six big e string
play the string¡¡, the pointer vibrates a lot ¡¡
then, it is basically a ligthwave system, at 10 eur the cheap optical mouse, can buy 4 cheap to get the optical sensors and led from they, they will do the basics for a bass pickup, the hard thing is the optical/sound circuit
some more ideas? this can be a good one open proyect