I've created an action where I set up constraints for cog joints. But how do I create an On-Off switch for it in User Properties or in the Pose Sliders window?

Maybe you should have done them in a pose rather than an action? I assume you know how to create a pose.....right click on model>New>Pose. Selecting ON/OFF and doing your cogs in there will allow you to switch them on and off in the pose window.

D'OHH!