Finally bit the bullet and did some coding this morning!
Followed Bimmercode's instructions to just have the engine on 'ignition' and not actually running. Kept the car on a battery charger.
Though strangely, once or twice my car seemed to switch off ignition by itself, back to whatever the other state is called "accessories" maybe? That would then disconnect the process partway...
Eventually managed to get a continuous connection (using the BLE Bluetooth one). Then coded the Drive position memory, auto stop/start memory, seat heat memory (but only found the driver's seat ECU, not passenger's), and rear tail lights with DRL
Did each coding separately, even those that were within Body Control Module.
I'd like to try adjusting the farside mirror tilt angle as it's currently dropping too low when I reverse...