Motorised Nautilus Gears #1

This is the first of two attempts at motorising MishaT's Nautilus gear set. (My second is here: http://www.thingiverse.com/thing:176585) In the past, I have used a rather expensive although very solid, geared DC motor for projects like this but have never had a go at 'hacking' a servo motor to achieve the same thing - low RPM, low voltage and a decent amount of torque.

The problem with using a servo motor of course, as well as doing the hacking, is that there is no immediately obvious way of