Couldn't you use a different washer or bolt? Granted exhaust pipes get hot, cold, wet, vibration, etc. but you could use a combination of different metals, fastener types, etc.
Besides, 75%+ of the times when I remove old, corroded, rusted, frozen heat shields the fasteners are going to break. After they break I'll try to remove them via drilling, heating, tapping, etc. Or I'll just use a threaded stud into the hole, heat and rust resistant washer and a nut. Most of the times any bolt that fits with some high heat loctite gets it done. I just don't over tighten and I know that it'll need to be broken to get to anyways.