Mechanical TV hobbyists make decent servos for the wheel speed when the optical detection pulses are at 400 Hz. I did one for my mechanical TV.
Besides the wheel inertia, however, it is much more difficult to make a decent servo for something at the low rate of 20 Hz that you have with the Col-R-Tel. I never was very successful with getting the 15 Hz mechanical TV wheel to phase up automatically, event though the speed was well controlled.
For what it's worth, the servo development is here:
http://www.bretl.com/mechtvprogress2...o%20design.htm
I think a multiple-pulse-per-rev servo for speed with an added once-per-red-field phase detector of some kind might be worth a try.