Rhindon:
The point is you have to save the UID by using events, so that they are saved AFTER the game has started, because the UID values are set when the game is run. You can't set a variable to a UID in the C2 editor, because the UID values are created afterwards, when you preview.
At any point you find fit (in the event sheet), you can use BodyPart.UID to get the UID of an object and save it to an instance variable you've created in the MainBody object.