Yet another stuck BB. Help me save my frame!
I thought about this a little more, and here is what I'd probably attempt:

The cup has a "shoulder" on the inside that retains the bearing cartridge.  Attack the shoulder somehow (break it, use your Dremel, et al) until it's possible to drive out the cartridge from the other end.  Then, with the cartridge out, is should be possible to pry the remainder of the cup out with minimal damage to the threads.