For our climber for Ultimate Ascent we used a ratchet wheel and pawl obtained from McMaster-Carr and used a solenoid to release it. With a ratchet mechanism it will wedge in place so you'll have to drive it up before a legal solenoid will have the power to release the pawl.
http://www.mcmaster.com/#gear-pawls/=vkbtc5 They aren't cheap and you'll need to fabricate something to connect it to your driven mechanism as well as fabricate a way to connect the pawl to your solenoid. Stainless is not as easy to machine as mild steel or aluminum. I do not know what solenoid we used or where it was obtained.