Well, I may have lucked out.
I'd cleared the VAG COM codes and the door codes came up again.
So, I went to inspect the wire loom of the left rear passenger door. I detached the boot and examined the wires. From the very narrow viewing zone it appears that most of the wires are intact. But I did notice one had insulation cracking, just one running perpendicular to the wire length. I couldn't see very well and wasn't able to detach the black multi-wire plug. I put the boot back and... the door lock works again! It must be a faulty wire and I just need to secure it back together.
Gary-A3's post in Question about A3 8P door wire loom repair was marked as the answer
