I worried about field rotation until I built my computerized altaz and found that field rotation can be ignored for the vast majority of visual and CCD imaging needs. So, I have never gotten around to implementing a field rotator.
I agree that building a computerized altaz is straightforward. However, there are numerous small everyday engineering (software, hardware) challenges that need to be solved for any new design.
May I point out my web page on how to build a simple inexpensive computerized dob at http://www.efn.org/~mbartels/altaz.html?
Mel Bartels