That's a very cool harp!
I'm also building a harp, but it is a framed one. I've finished a rough 'Blue Peter' three-string proof of concept using a single red laser pointer, a diffraction grating to split the beam and three LDRs to sense the beam. It's ugly but it actually works really well.
I'm building proper octagonal frame for a ten-string 'mesh' style harp - two sets of five strings running perpendicularly across each other. The idea is you should be able to play up to four note chords with two hands... and it's a bit different. I'm just waiting on the LDRs I've ordered, and I should have a nice new instrument!
I'm using ten separate 1mw laser pointers, because that's all I can afford for this project, so I'm expecting to have to use a lot of smoke for it to be at all visible...
I'd love to build a scanning frameless harp too, but one look at the cost of the scanners sent me running...
One idea, maybe you could couple your 'brightness' sensor with an IR distance sensor, to get more reliable continuous controller info dependent on your hand position?
Dave