This might be obvious to all but it took me a week to figure this out. When I want to work on the Coup Perdu clock for any reason including changing batteries, the easiest way to "disarm" it is to pull the pin indicated by the red arrow in the photo below. Remove the lever with the magnet and the motor is permanently off until it is re-armed.
Good point. Thanks for pointing it out.
I usually use a piece of painter's tape to hold the lever in the open position when travelling. Removing the pin is a much more secure way of disarming the rewind mechanism.