I have no experience with putting salt in water during a fast. Personally I wouldn't do it. I would always assume my body has the reserves to call upon if needed. Nor in all of my research on cramping have I found any evidence to suggest that salt is definitely the cause of leg cramps. I'd suggest they won't last too long, and the best remedy, even if it is only short term and still painful! Is to stretch the leg when you feel the cramps coming on.
Sorry I can't be of more help with this. And Thank You. :)