Capture Nut Repair

I posted this video on youtube the other day seeking advice on how you would fix this seized bolt in a capture nut. This bolt secured the vent window to the door frame, so having some movement in the mounting is why a capture nut was originally used. The seized bolt caused the square capture nut, however, to break free from the cage and began to spin freely, so there was no option but to drill out the head of the bolt to free the vent window assembly. Now that the parts are removed, I needed to fix this broken bolt and free spinning capture nut. Normally, I would cut cut open the cage, remove the rusty nut, and insert a new nut and weld back up the cage, but the back side of this cage is hidden by the outside door skin and it’s in a very inaccessible place. I had some suggestions on YouTube, but I’m always open to more suggestions. If worse comes to worst, I can always cut open the door from the outer door skin, repair the capture nut cage, and have my body guys repair the door skin damage!