On my previous pop-up trailer I put a roof rack on my trailer, I do believe that it would probably void the warranty as I had to drill through the roof, but honestly I didn't even think of it at the time. The roof was made of the same materials as seen in your pictures. The roof is a fiberglass overlay laminated to what appears to be a 1/4 in sheet of plywood, on the interior there appears to be another sheet of plywood, the two sandwich the framework, and are filled with a hard foam insulation (seems like the expanding foam squirted between the layers then hardens)
The rack system I used was Thule, included the roof rails accessories that can be installed on Leer truck tops, then using accessories I was able to haul three kayaks on the top, or 6 bikes, or a combo of both.
I don't have close up pictures, but this is what it looked like loaded. It worked very well, all the screws installed where sealed with silicone caulk as was the bottom of the rails as I installed them.
I am not saying this is/was correct and appropriate, but I can say I did not have ANY adverse problems, and it worked very well for me. Loading was simple due to the height of the trailer, the weight seemed very well distributed using the Thule system. Just of note, it is difficult to find the roof rails on Thule's site because of the vehicle fit program they use. I bought the whole thing from rackwarehouse.com. Just look through all the Thule accessories or search for rails.