Here is a quick thought of the idea that popped into mind this morning for the mannequin you are working on.

Forgive the horrible art skills,

But, you can get the idea of what I am thinking by looking at E1 and E2 and using that in place of the discs used in the image.

E1 acting as a claw/brake by its teeth locking into the gear E2 and holding it in place.

I am assuming, a small simple cheap servo could be used to control the claw to lock gear into place if the teeth are set in a way that there is a slight hooking action to prevent slippage.

It's a simple thought.

Ok, i will let this be my last thought on it for a bit while I go back to working on other things. But even a cheap linear actuator put behind you servo or somewhere convenient to push an arm with an end shaped like a crescent moon with teeth to push toward the E2 gear locking into the teeth and holding its position. Covering a wider area of teeth and preventing slippage.

I believe I have come up with a much simpler servo break. I have not tested it yet but have ordered a couple of the solenoids from sparkfun.

the idea: I will 3D print a cog that will go on the servo shaft, the top will be the same size as the shaft so as to still accept what ever needs to attach. I will also 3d print a end piece for the solenoid shaft that fits into the grooves of the cog. When not powered the solenoid keeps the servo from turning, when powered will pull back and allow the servo to turn. Link to the Solenoid: https://www.sparkfun.com/products/11015#documents-tab

My only concern is, will the solenoid be able to pull back when it is holding a weight such as an arm elbow servo.

Place a command in your script to raise the servo just enough to take the weight off the pin then pull it back with a second command.

I thought of that after sending the post. For the cog, I am going to try and use the Nylon drive gear from an old servo. Drill the bottom then use heat up a brass drive gear and press it in to make the splines.