I read in an article by Bruce Barnbaum that to save damaged negs like that, he would go through the painstaking process of printing the negative, damaged, warts, and all to normal perfection standards. Next step is to retouch the print so it looks like it didn't have the scratch. In this case, that would involve very selective bleaching.
The best aspect is that if you mess up, you still have your original negative 'intact'. It may involve problems if it's a very large format neg. 4x5 and smaller can be enlarged to larger size, which would be easier to 'repair'.
Then photograph the print. Huge job, but could well be worth the effort if the neg is worth that much to you. I think it would be easier than to spot the neg.
Of course, I've never tried this myself, so no first hand experience.
- Thom