Lowering a pop-up TC....is there some trick to this?

With that type of lift system lowering the top is a two person job.

What you might try is making some type of adjustable pole with a notch at the top that you can prop under the handle at each point you want to stop.

One problem with that though is most cranks are removable (I presume yours is as well) and they can pop out of the socket pretty easily if pressure isn't applied to hold it there. So you are not only looking to stop the crank from turning but also to push inward on it.