Hello,
sorry, I have no information about VWs, but I think you’re on the right track.
The Martin Walter roof on my Land Rover had fiberglassed into it factory-installed steel reinforcement plates at the hinges and steel tubes oriented into an L-shape along and across the roof, respectively, all of which were completely rusted away to dust. See near the bottom of this web page:
http://n.ethz.ch/student/jhackney/lifting%20roof.html
I replaced the plates with zinc-plated steel and I replaced the tubes with aluminum ones and restored the fiberglass. The roof is rather flexible, but that’s the nature of fiberglass: it is plenty strong even if it isn’t stiff.
Good luck!
-Jeremy