I'm not convinced the rechargeable cell approach is worth the bother - life might not be much longer than a primary cell. The capacitor approach seems better, though for sizing I would note that (at least with my Model S) it's often necessary to press the button more than once - first press brings it out of sleep (audible clunk from contactors in the car), second press a second or two later actually opens the port.
Messing with the CP does have the extra concern that faults in your circuit risk interfering with the safety function of CP, though you can no doubt engineer it such that this is unlikely (large series resistor both in value and physical size so there's little risk of it being bypassed by mechanical damage).
OTOH, I have the port open button on the wall by where I hang up the connector and find that this works fine from a practical point of view, avoiding this difficulty.
Messing with the CP does have the extra concern that faults in your circuit risk interfering with the safety function of CP, though you can no doubt engineer it such that this is unlikely (large series resistor both in value and physical size so there's little risk of it being bypassed by mechanical damage).
OTOH, I have the port open button on the wall by where I hang up the connector and find that this works fine from a practical point of view, avoiding this difficulty.