The meat of the rear racks situation is that mounting any significant weight to a bumper is beyond the design of the load carrying ability of typical trailer bumpers and secondly, any weight put onto the extreme rear of the trailer frame is changing the load carrying design of the trailer.
Some folks may have a situation where the tongue weight is greater than the design applications and adding weight on the rear may not be as consequential, as long as the frame has the ability to carry that weight without significant frame bending moments. The change in load moments is a difficult equation to discern because changes in the trailer attitude and road surface are brought into play. IOW, a frame that is at rest may be able to sustain the weight of rear load stationary moments but suffer failure from the extreme loads while on the road.
It's a pickle and should be avoided unless an owner likes pickles.:)