My recommendation would be to use a ratchet and pawl. These parts are easily available from many gear companies including Berg(
http://www.wmberg.com/catalog/catres... oup=RATCHETS), or if you're on a really tight budget you can probably get creative with a band saw and a grinder. I would be very hesitant to rely on a mere servo to disengage the pawl. However, something like a pneumatic with a very short throw would be suitable.