[JeepForever]

Assuming you are correct that there is no possible way to access the hole, I would get solution oriented try one of these options:
1. Cut a square access hole to fit a nut and wrench in (I know you don't want to cut, but this may require desperate measures)
2. Drill a small hole near the nut and spot-weld the nut back in from underneath (repeat on a couple corners) - also not desirable because you may weaken part of the frame)
3. Find some way to get inside the frame (either by cutting, or taking parts off), slide a box wrench in to hold the bolt, then crank it down (remember to get the wrench out) - I did this with my back bumper when a nut spun while installing a trailer hitch)
4. Create an opening and drop in "flag nut" - a nut welded to a plate that's large enough that it won't spin (acts like a giant washer and wrench).
I know my response is a bit late from your post. Let us know what you came up with.