I don't know if you've resolved this yet, but I ran into a similar situation the evening before last. My captured plate isn't stripped, but it popped off during reassembly of my body (I pulled all the body panels except for the tub when I painted). I remembered seeing this post, so I figured that I'd document how I fixed mine.
Initially, I tried to fish the backer plate back into position through the hole that is under the plate that ties the windshield to the dash on the inside. I tried everything from extension magnets to flex handle magnets, screwdrivers to bailing wire and only managed to get p*ssed off in the process, so I took a little bit more aggressive approach. (This method should also lend itself to get to the side that you're having problems with.)
With the windshield folded down, remove a few screws holding on the cowl gasket and pull it back. If your gasket is bad, this is a great time to replace it. Then, make a "Z" cut (see pic). I just used a 4 1/2" angle grinder with a cutting disk. I don't recommend getting as close to the end screw hole as I did, but it didn't cause a problem.
Then, fold the two triangles back and remove the irritating piece of metal.
Now you can place it where it belongs, start a couple of the bolts, and (in my case) bend the retaining tab back down so that I won't have to do this again.
Now all you have to do is fold the "Z"s back in, tack weld a few spots, clean off all the metal and put a little paint on (I had just put my windshield glass back in and didn't want to get paint everywhere, so I just used some glazing sealant as a rust inhibitor). Once the paint is dry (or in my case, sealant), reinstall the cowl gasket and you're good to go.
The nice thing about doing it this way is that it's completely covered by the cowl gasket. It shouldn't cause any leaks or ever be a cosmetic concern.
I hope this helped.