Is there any way other than waiting for a thaw to free up a rear wheel that is locked due to freezing following a car wash and e brake?

Rear brake frozen. Assuming you do not have a heat source, small heater to aim at it for a while. If not that, I would think you could use some windshield deicer. I assume you do not have disc brakes on the back. There should be an access plug on the back of the wheel plate. On the back side it is usually an oval rubber plug at the top or bottom, varies car to car. Spray the deicer in up or down and I would think this might do it. Depending on age of vechicle, the cable running to the brake may be the problem.