I've been out of high school too long to answer this one ...
This page does a good job of explaining the tangent error that you get when using a threaded rod to drive a barndoor mount (and the resulting tracking inaccuracy):
http://www.mikeoates.org/mas/projects/scotch/
What I'm wondering is whether a threaded rod with a universal joint or CV joint in the middle of it might solve the tracking error. Eg, each half of the rod would remain at 90 degrees to its arm, and both ends would be rotating in a threaded nut mounted in each arm. Maybe the rods could be joined in the middle by something other than a universal joint. As the arms separate, the universal joint should stay half way between them, with the angle between the 2 halves of the rod gradually approaching 90 (from starting at 180 degrees).
So, the question is: would this give the desired constant angular rate of change between the 2 arms??