To the best of my current knowledge, bedded aftermarket glass installers prefer bare steel as a substrate. They use a special primer on glass and bed, for maximum bond strength with the urethane adhesive. There are even some rules / guidelines for insurance work within the collision repair industry about whether epoxy primer can be on a glass bed.
Because if the windshield is only stuck as good as paint, it might jettison in a wreck and not provide the structural support it is designed to. Now, in light of that news, consider the fiberglass vs metal case knowing as we all do about the porous nature of fiberglass and other polyester products. If a guy was to hunt around, I don't believe he would find any OE design not using a metal windshield bed.
There was some C5 body stuff included in some GM training I did years ago and that birdcage, by then, was pieced together from a few different special metals. I don't know if anybody ever even used aluminum instead of steel there though. I built a Model A coupe chopped windshield bed not long ago and it ended up being a single layer of 18 ga. CRS but I would prefer two, or a 16 ga inner and 20 ga outer. Just my .02 on that, and I'm just one person.