There is a 4" high by 12" long by 1" thick backer board behind where the hooks are installed. I was just at the factory and asked this same question because I added a hook on my old PC. I did not hit anything with that hook. Bobby and I went looking at how the rear end cap is make and mounted. I want to put a hook in the new PC but have not figured out how just yet.
Tom, I called and talked with Bobby today and got the same info, backing board is only where the existing hooks are, thanks for your confimation on that.
I'm not anywhere near my PC right now, but I was wondering if you could simply cut a 4"X1" board to the desired length, bevel the ends for appearances, and simply glue it with some sort of really strong epoxy or mastic glue to the inside of the rear cabinet, paint it white to match, and then just screw your hooks into that and on into the fiberglas beyond. Unless you plan to hang something very heavy, that ought to do the trick and it is out of sight inside the compartment. Just a thought.
Jerry, that sounds like it might work, I would screw the board to the fiberglass as well as glue.
Also, instead of a hook like the factory ones with one large screw attachment, I might try to use a hook that attaches with a couple smaller screws. The smaller screws might hold better in the fiberglass since they would be in shear rather than bending.