[ksafin]

You may want to consider doing something along the lines of what the guys at iR3 are doing for Robot In 3 Days.

They're basically using a gearbox with a shifter that shifts between an engaged gear (for rotating the winch) and disengaging from the gear into a neutral position for release of the "catapult".

It's pretty straightforward if you've worked with shifters before, and as far as "custom" goes it's not too custom, there exist parts that make it pretty simple.

It's probably easier to do than modifying a COTS winch to your specifications.