You're replacing the strut insert, right? If so, then yes, you can heat the remaining threads. (Heat the b'jesus out of it)

My experience with these has been that the nut takes a tremendous amount of torque to remove even when it's completly intact. I'd be surprized if you could get the amount of torque necessary with the top of the nut broken off.

If that's the case, you may have to resort to major surgery. Just off the top of my head: maybe use a Dremel to cut the remaining top of the nut out so the strut insert can be removed. (will the strut insert fit past the remainder of the nut?) Then use a hack saw or Dremel to carefully cut through the inside of the threaded part that's stuck. Maybe cut it several places in an effort to compromise the strength of it.

Good luck.

--
Jeff Pierce (Post back with your results... it's what makes this forum work.)