Life got me. Busy summer, who knows if it's over yet...
I ran across a website a while ago about using a laser and photodiode to track a target in 3D.
Now, they use a pair of galvanometers to control the laser. Nifty. You can build some yourself actually. It's mostly a magnet, glued to a shaft on bearings, over a magnetic coil. One end has a first surface mirror, the other has a feedback mechanism.
One issue with building galvanometers yourself is that you probably won't get anywhere NEAR the frequency response of a pro set. I know of a pro set for laser shows that runs $700 or so. That's less than half what most places would charge.
So, do we have alternatives? I believe yes. At the loss of flexibility and speed, we can probably use two standard RC servos for X-Y positioning. The high speed circle drawing could be replaced by a small motor with an off-kilter mirror. Now, to adjust circle size, either a small push rod to another servo-esque setup (worm gear?) or a different focusing mirror arrangement, I'll have to see what's pratical. A stripped laser pen diode should work fine for the source, and I can get a decently matched photodiode from Digikey et-al. Optics, motors, and other oddball stuff actually should be available locally for me at American Science and Surplus. Even the parts for a first run galvanometer if I so chose that route.
While the demonstrations they do are good, they have limitations. Power output and circle size is based upon relfected light from the target, and it's assumed to be roughly spherical. Makes adapting it to track other objects difficult. Not impossible. I'm more worried about tracking, say, my pasty white finger and then having one of my darker compatriots try it and the system thinking he's a meter farther back than he is. Time will tell. Maybe a slightly off-axis reciever running a 2D PSD (position sensitive device) would allow for rough triangulation and automatic tuning.