I've been thinking about this for some time myself. I was looking at it being friction driven on both the alt and az rather than any sort of gearing. Main reason being that you would still want free motion for the larger slewing motions without having to use a clutch. Of course anything adding friction would make it harder to locate the scope by push/pull but then I would have the fine motion controls to do this.
I was thinking of something like roller blade wheels for this purpose, either on a worm drive or spun directly by the cable. Now that this thread has started it up I might stop thinking and start doing.
Trav