Nic, good point, we have a few custom shops here....
Looking at it very carefully again this morning, is definitly not structural where they have cut and the rest of the car lower down is immensly strong, you can see the separate strengthening structure. it also rides on a subframe.
I also had a look at some track cars (E34's) where they have removed the rear shelf to save weight and the roll cage goes straight through that area.
So:
1) Fibreglass.
2) Rivet an aluminium overlay in.
3) Strip car and weld, probably not worth the effort.
4) Carbon fiber patches?