I could only find one other post on this but it's useless telling you as no answers were given, I suspect it's a closed box section unless someone's had a wing off and can say different.
Off the top of my head, have a look to see if you can figure out how much of a flange is inside the box section, evidence of spot welds, mirror, endoscope? whatever. If there is enough flange inside you could then try drilling thru the A post and the flange behind and pop riveting it (not weedy rivets) back into place. You will have to hold the plate somehow whilst you drill thru, perhaps a molegrip or similar will do the job? This will no doubt involve releasing the check strap from the door, if you do put some stiff wire thru the hole in the check strap mounting WHATEVER you do don't allow the possibility of it falling into the box section. It may also be possible to weld it, I know diddly squat about welding but I'm sure it will bugger the paint on the A post.
Anyone get any info re box section or a better idea?