I heard about an electrical solution to (this on a '50s custom car). They hid a 9V battery connector in the car's grille, wired in parallel to the driver's door solenoid.
As long as the bad guys didn't know what it was for..... and you can get hands on a 9V battery, it was enough to open the door with a dead battery - or even no battery.
Still it would be a good idea to have a mechanical solution.
Why is it "almost" finished, you ask?
Well, we realized that a failure of the battery cutoff means there's no way to get into the car (assuming it was locked) and that would be kinda dire, so we need a secret/hidden, external manual rear hatch release as a last chance option. We already have an interior cable release for the hatch and I'd like to maintain it's functionality as well, so it could get tricky, we'll see.