I find it very strange that folks think it's proper to relocate a plate from the registered vehicle to which it was assigned to the cargo it's carrying. I suppose they are likewise installing license plate illumination on their long loads of lumber?
I read about the Michigan trailer ball in the step bumper case, where they ruled against the motorist. They carefully avoided the question of what if the motorist were actually pulling a trailer at the time. Not all trailers need to be registered/plated.
I live where a TC is registered and plated like a vehicle. BF placed the plate holder in a location where it is partially obscured by the ladder, and the scissor stairs block the truck plate pretty good. Haven't been pulled over yet.