I suspect if I did the repair with epoxy/glass, then took it to them and just told them it had been welded, they'd do it.
Here's an interesting kit I found - similar to fiberglass (in fact that backer material is probably fiberglass. One thing I'd want to do is use por15 before and after the repair just to be sure
No Weld Hole Repair Kit
I've been researching both flux core welding and brazing for body repairs like this, and both seem to have potential for causing paint failures if any flux is trapped in the weld and works it's way back up at a later time.
I have only found one other shop locally that's both returned my call and has glass for the car, and they told me that they would epoxy the hole for a reasonable price.. Then they told me that the windshield on my car was glued in according to their system...so yeah, they're not going to get the car.