So, this is in the same vein as the Unpluginator. It's sole purpose is to remove it's own power source. Due to the time required to put the batteries in and sit upright, this needed a button to trigger it. I used a 555 timer (27p for 10, including 7-day delivery from China, insane) to create a latching circuit. This triggered a relay, which turned on the motor. It could have been more refined, but the only spec was for it to work. 

The motor is unable to pull the latch open when already taut, so I leave some slack in the string. This gives the motor time to get some momentum, pulling the string tight with enough force to pull the latch open.

Initially I tested it using an external power source, before attaching the circuitry to the device.

I could design a circuit board for this, but the proof of concept is there and that was enough for now.