I've been reading a lot lately that says that once a cramp starts, you can stop it by taking something very sour, like vinegar or pickle juice. They're not sure how it works, but the thinking is that the strong flavor somehow short-circuits the nerve signals causing the cramp.
I haven't had a chance to try it yet (and I'm happy if I never have to!) but I swiped a couple of packets of mustard from the condiment counter at the movies a few weeks ago
, that I've been carrying on my tougher runs, and I plan to bring some to my next marathon. Last time I tried carrying crystalline citric acid, but my packet sweated through before I had a chance to use it. 
Good luck - hope you find something that works.
Speed comes from what you put behind you. - Judi Ketteler