Thank you for those photos (door skin fixings). I never thought of doing it that way.
My original plan was to screw 'L' brackets to the steel door skin with captive nuts in the L bracket and screw the GRP door skin to the L bracket. In my case that would mean my screw head is in the door gap between the door and the B pillar where it's too tight for screw heads so the screw head would be recessed below the level of the GRP with a plastic cap over the screw head.
But I like your idea too so maybe a combination of both. It'll be a while before I get to that stage though but I will post pics on my build thread when I get there.
It takes a bit of jiggery pokery but it works. Good tip i learnt by doing it ia drill the holes through the door and skin for locating them rivet nuts. Make the rivet nut expand using the tool but not in the door, just on the tool. Then make the holes in the door skin a lot bigger than the squeezed rivet nut so its not tight in the hole.
Then put the screws through the door and rivet nut on them like the image below. When all tight put fibreglass filler into the holes in the door and lift the skin onto the door panel and clamp it
Then after a short period 20 mins or so gentlely loosen the screw a bit so they doesn't set against the filler. Leave 24 hours to set.
I did it this way in the end after several hours previously trying to get screws to aligh with the rivet nuts in the doors to no success. Im going ro do the other door the way I finally did as above and it will save me hours.