Martin, I have a Dremel, and even thought about using it, but unfortunately, the bulk of what needs to be removed is beyond the reach of the bits available to me. There is a 7-8mm counterbore, and the threads continue another 30mm. The rat tail file looks like the only, albeit, time consuming, option at this point.
With an unevenly tempered hardened bolt broken off in an aluminum block, I am increasingly becoming aware of the fact that there is a Helicoil in my future.