Interesting project Shobley.
Do you know if there are any RGB laser harps out there?
If you are up for invention/innovation you might want to transform that timing problem into a spatial problem by using any old webcam as the detector. Fixed right above the scanhead, all laser beams will appear parallel (i.e., vertical lines) and detection of a 'pluck' is a matter of finding the horizontal coordinate of the extremely bright spot. Wouldn't know if this can be done by hacking into the hardware of a webcam but software wise it shouldn't be too hard. There are some artificial vision programming libraries available online that should do the job.
Once you have that it shouldn't be to hard to turn a RGB projector in a most sophisticated laserharp Since the whole thing is computer controlled anyway imagine colors changing with the music etc. Could be really cool.
Let us know how your project progresses.