Peter, I've heard of something that people do for that sort of thing which may be a kind of deconditioning. They put a rubber band on their wrist, and whenever they catch themselves doing it, they pop the rubber band painfully on their skin.
I tried the rubber band idea, Moonbeam, and that has been working very well, better than my finger pinching. It still gives me some tactile way to channel my nervous energy, but without nailbitiing. Now my fingernails are starting to look normal. I still get the urge, especially when I'm stressed, but now I just ping the rubber band.
So far, so good. Thanks for your suggestion!