There's a discussion about electrolytes going on over in Endurance/Trailriding, check it out.
If your horse does not drink, be careful with electrolytes. The horse needs to be hydrated to make them work. They are supposed to replace lost electrolytes after a hard workout, I wouldn't give them just to make the horse thirsty as you are suggesting.
There were some other threads about teaching your horse how to drink away from home, using flavored water and so on. Hopefully those will come up when you do a search.