So far I'm thinking that I'll need to grind the bolt heads down, then drill through the remainder of the bolt. The bits that fall into the frame from this should be small I assume and I can get these through the holes in the tubes. After that my plan would be to try and drill the riv nut at the sides or filing them down, trying to remove a section so that I can try and fold the remaining riv nut and pull it through the hole, leaving nothing in the frame.
Any thoughts on whether this approach will work? In theory it should, but in practice I'm sure it will be more tricky than the simplicity suggests.
You'll have a job drilling the bolt out, as how will you stop it rotating in the frame? The same goes for drilling out the rivnut as it will rotate in the frame too.
I still reckon that grinding or filing off both the bolt head and the head of the rivnut, punching the remainder through into the frame and, if it can't be removed, sticking it in place to stop it rattling is the best and maybe only way.
It's worked for me, anyway.
Or, if you have access to oxy-acetylene or oxy-propane, assuming the rivnut is aluminium, you could always melt it out.