Hydration is one of the main reasons I feed beet pulp. It soaks up SO much water, I know my boy is "drinking" at least a bucket-full. You have to be sure to let it soak until there are no hard pieces left. I didn't appreciate exactly HOW much water it soaked up until Christmas Eve, when I decided to soak it at home before I took it out to my boy. If you don't mind the smell (I actually like it, but that's because it's a horsey smell to me!), you could even do kind of like I'm doing and soak it at home/in your car while you're at work, then it's ready to go by the time you get out to your horse! My horses will eat as much beet pulp as I give them, plus I add any supplements to it that I want.
As I noted later in that thread, I do beet pulp in the winter for exactly the reason you're describing, as well as in the summer to keep him hydrated on endurance rides.