@tsl: I once had a good idea on how to produce perfectly straight compound radius fingerboards. It is a method often used for shaping the profile of RC Plane's Wings. I did not try the idea myself, because after thorough research discarded the idea of a compound radius fingerboard and went with cylindrical fingerboards. The idea works like this:
1. Make metal templates for the both ends of the fingerboard, which have the desired radius and shape you want on each end.
2. Take a long fret leveleing block or a long and not too wide sanding block and sand along the run of the strings, until you have sanded the wood down to the metal templates on both ends.
Greets,
Marcel Knapp!