I've had luck using some JB Weld and letting it cure for a few hours.

 

Heat didn't work, I'll have to pick up some jb weld to give that a try.

Thanks for all the ideas! I really don't want to have to break out the die grinder

 

I agree with Resharp, I would cut the loop of first and then the top portion of the tap that holds the loop. Then you will be able move the rest of the tab to get at the Rivnut to get a better hold with visegrips. I would suspect you are going to replace the Rivnut anyway