I'll just point out that a 180° retract servo is available, which requires no current draw when locked in either direction. As David Agar alluded, this requires that your setup is precise, or the linkage has an overtravel feature, because the retract servo only has two positions - full one way, or full the other.

You will also notice that your retract channel (at least, on every radio I've encountered) does not provide proportional control or adjustable endpoints. This does not lend itself well to the conventional proportional control servo. I suspect if you were to use an ordinary proportional control servo, you would want to connect to a spare proportional channel.

Good luck,
Dave Olson