Hello helpmonkey, and welcome to the forums.
I don't know enough about electronics to be of any help there, but I have seen a couple of other people who have done something similar to what you are trying to do.
You might have already seen these, but, if not, here they are.http://www.foundry.ray-vin.com/fusion/fusionstory.htm]
Both of those sites use a two axes system instead of a single axis. Are you planning on making two single axis trackers for your heliostat? I think that would add a lot more capability. A single axis system would only really work well when the heliostat is aligned a specific way using a Polar Tracker setup. (Here's a Wikipedia link which explains the polar tracker some (scroll down). http://en.wikipedia.org/wiki/Solar_tracker
A single axis system would work, but a couple downsides are that the target must be situated directly in front of the heliostat (in line with the tracker's axis) and that the heliostat's declination would have to be changed manually as the sun get's either higher or lower as the seasons change.
You might have already known all that, but I figured that I would say it anyway in case someone else is interested.
By the way, is the Greenwatts site yours?