It is possible to to drive a tiller using a winch drive.  I have success with this using a 1 inch pipe mounted to a windshield wiper motor.  A line is wrapped around this pipe 5 turns, and connected to the tiller.

The alignment of the pipe is critical to ensure the line does not walk to the ends.

The advantages over tillerpilots or linear actuators:
   * use windshield wiper motors from cars, available for very cheap.  These motors also have two speeds.
   * easily adjust the gain by using different size pipe (recommend 1 inch or smaller diameter)
   * reuse existing line connections from windvane
   * testing shows this is more efficient than using a tillerpilot

The disadvantages:
   * Slack in the lines gives more play in the system
   * More work to connect/disconnect lines than lift tillerpilot from pin
   * More involved mounting to the boat
   * lines across cockpit