If you have the capability to do fiberglass then the sky is the limit for you!!

With that ability you can build anything you want, so yes I see no reason as to why you couldn't just rebuild the door panel to any way your heart desires. It's been done before by others in a multitude of vehicles so this would be no different. If you build fiberglass sub boxes that are holding up to that abuse I doubt you'll ever have any issues with the fiberglass on a door, ever. The subs are putting far more force and abuse on the fiberglass in those enclosures than you could ever give it on that door panel aside from having Jet Li or Jackie Chan doing their best kicks directly at it, lol.

Go for it man and be sure to post pics of the work, I know everyone would love to see it as much as I would. I wish I had more experience with fiberglass myself. I'll be diving into that world soon enough though as I'll have to do similar work to mount the full ranges into the sail panels and now to fit the 8" midbasses into the doors.

Yay, fun for both of us! lol