I tried (carefully) to simply break off the bolt...no luck, and I don't want to deform the sheet metal on the hood itself. I cannot cut it, as the nut is tight up against the bottom of the hood (no space to put a cutting tool). And PB Blaster probably won't work: it looks like the nut is completely fused with the bolt, and the bolt itself is flush on top with the flower-shaped disk on the outer surface of the hood. It just spins, with nothing to grab onto to try to force the nut to turn. Damn.

And yes, the metal underneath the insert is effing disgusting. Dirt and gunk and bugs and gross stuff. I guess roads are dirty places to drive. Dirt roads, too.