I got a stuck rivnut out recently. Hacksaw and about 30 seconds. I had previously removed the head of the bolt so I could remove the cable guide. After that, got the saw blade under the lip and after about 30 seconds it was free. I'd angled the blade so as not to cut downwards and damage the frame. Rivnut dropped into frame leaving a nice hole to full with a fresh one.
Getting the remnants out of the frame is my next challenge as the frame also has a stuck crank arm so I can't get the BB out. Not taking a Dremel or anything else to m950 cranks and BB though, I need them to survive any procedure. Damn the previous owner who rounded off the crank bolt.