AWOL:
How are you doing the drive coupling?
I was imagining something like the split-band drive around the focus ring.
Split-bands were used on old HDDs which had stepper motors to move the heads before voice coils became common.
Whoops! Have no idea what you're talking about... 
Just to make sure I'm explaining my self correctly:
The setup is very much like the "Knob" sketc, plus added power to the servo (using Vregulator and decoupling capacitors).
The code was modified to fit certain needs (recording and playing back...).
The servo has a cog wheel attached to it, and this in turn fits to a ring that wraps around any lens I use.