The challenge I see is if the motor takes too long to stop moving, the hand could move past the desired stop location, and keep going. Worst case you could make an electric brake circuit, like cordless drills have, that will stop it more or less instantly when the input signal goes away. But hopefully the worm drive motor will stop fast enough as-is. I'm not sure how quickly a continuous turn servo would stop when the power is removed, but that could potentially be another solution. Anyway, neat project, it will be interesting to see how it works.

Im hoping that if the motor runs slow enough, the chances of over-shooting will be minimal

 

Im hoping that if the motor runs slow enough, the chances of over-shooting will be minimal

My test with this motor indicate it won't be a problem.
The placement of the sensor under the clock face isn't critical.

 

The simple way to stop a DC motor from coasting is to short-circuit the motor terminals when it is switched off. The stop will be almost instant.