Just to add an idea. I decided that rather than hacking it up with a chisel, hacksaw, dremel, etc., to just take a cutting torch with a welding tip on it. I added enough heat to get the head of the screw cherry red, but not heat the strap. Waited until the screw head cooled enough to not show any color, then used the screwdriver and backed the screw out like it was in butter.
I would say you could probably have good success with a MAPP torch similar to what a plumber would use to sweat copper fittings. They sell them all over.
