I'm not sure that it is. I know that there are sockets that have a switch in the lid, but that's mostly for turning off the fog lights as far as I understand.
The feature for the vehicle to recognise that it has a 'trailer' attached is for its plug to have a link between pins 12 and 13 to ground pin 12 when something is plugged in so that the vehicle loom can recognise it. It could be that my bike carrier does not have that link, I will check at the earliest opportunity.
I know for sure that the tow bar loom interrupts the rear sensor loom as there are 2 connectors that go in between the original body connector and the bumper connector, so the feature ought to be there. A couple of times that I have been into Reverse the sensors have been automatically disabled so perhaps I just have an intermittent contact somewhere.