Well I ended up cutting the chrome coachbolt head off in pieces.
However, that still leaves me with the bracket half off.
dannyboy wrote:there is a nut on the outside but a square headed bolt on the on the inside that sits in a captive housing
Given that it's a sealed box section, how do you know that? Have you cut open the box section? I have the nut off the forward bolt which won't push back into the box because it's too long & I can't pull it out to get the nut back on.

I suppose I could try pulling it out as far as possible with needle nose pliers & use a Dremel to cut off as much of the bolt as possible. Then the head would drop back into the box & I could drill through the other side of the box section & use a long bolt through the whole box. I'm surprised BMW didn't do that in the first place.
I'm sure bodyshops must have got around this problem before.