Thanks everyone for the input thus far. I really appreciate the collective brain trust in action.
Colonel J: Yes, I did cross-post in the framebuilders forum here.
So far, the only recommendation from over there is from Calstar, who says to file away just enough from the damaged area as I originally suggested:
I think your best bet is to file as you suggested, just enough to be able to get a bb threading tool in to chase the threads. I doubt the bb will need facing, dent is probably not disturbing the bb face plane. The slight loss of full depth threads in that spot won't be a significant mechanical problem. I would not try to bend the bb from the inside or outside, only bad things will happen. IMO for all that.
I think the 20grit's idea to use a slotted BB cup may have some merit. However, it may be a lot of work to cut up an old fixed cup and then I'm still not sure I could get it to thread into the shell while the dent is still there. Hmmm...
So far, I like unworthy1's suggestion to start with a regular BSC tap and hope that it can re-cut the threads deep enough to screw in the fixed cup. At least that option is not as likely to cause any further damage. Then I can move onto more drastic measures if that fails.