My understanding was that Canada Water, Wapping, and Surrey Quays were getting platform extensions, but not Rotherhithe.

Where there is selective door opening it may well be that the last carriage will not have doors that open, the avoids moving the signalling equipment (and driver) into the tunnel. But one of the great things about Overground trains is how easy it is to move from one carriage to another as you approach your destination. Not that this would be suitable at Canada Water, the busiest station on the network, but it would work at Rotherhithe, which is much quieter.