Off the top of my head, the sensor could simply be the lever on a microswitch- read the switch to decide what to do.
Then using an h-bridge for the motor you can handle the direction and (if I understand these things correctly) also brake the motor electrically rather than using a mechanical brake-shoe kind of arrangement. But I have no idea what kind of breaking force can be applied electrically....
I also see, btw, that Pololu have these sail-winch servos Pololu - GWS S125 1/2T 2BB Sail Winch Servo
Jim