Regarding the backup battery, is it a rechargeable unit on the OEM install on Turbos?
Looking at the schematics I would say it was a rechargeable battery.
I think a battery would be my prefered option as it would allow you to get in if you where out somewhere with no access to a charger. But then again if you have no access to a charger then what use is getting in so either an external release or some accessible wires connected to the battery so it can be charged easily.
I guess that is the right way to go.
Dead battery = stuck until jump started/replacement battery installed.
Would be nice to sit in the car if it is raining though. /worst case scenario/
The easiest way to ensure you can get into the car with a flat battery/broken control motor/dodgy central locking relay is to connect two wires to the lock/unlock wires that go to the motors (usually green & blue wires) - not the lock/unlock command wires from the module.
Run these wires through to the bonnet area, up through the plastic scuttle panel by the wiper arm, and along where the wiper blade sits and fit a waterproof terminal. If you leave them tucked under the corner of the bonnet you'll be able to fish them out and apply 12V to them - the motor(s) will then directly unlock regardless of flat battery or faulty parts (unless both motors have packed up or you dont have access to 12V!).
The only surefire way of ensuring you can get in is to fit a mechanical device attached to the same point as the locking motor in the door that pulls the lock to the open position when pulled from outside....