I simply painted the plywood and then put my bedding down over it (in my case, sand). I don't have a moisture problem in my coops since I use a poop tray under the roost that I clean out every day, and use a repitle litter scoop to pick out any stray droppings from the sand on the rest of the floor.
There's nothing wrong that I can think of in using a fiberglass panel other than the cost for something that you probably really don't need because you're going to be putting some kind of bedding over it anyway, right? I would think fiberglass would be far too slippery for chickens to walk on without bedding over it.
Actually, it might be more worth it to put a fiberglass panel on the wall in back of your roost in case of poo splatters so you could more easily wipe them off.