Have her vet checked. I had one old timer that started doing this and he had cancer in his intestine. Not saying that's what it is, but your old girl deserves to be comfortable and I can imagine she's not feeling good right now.
Now, I don't know why it wouldn't work with horses, but when our goats or dogs get bound up I feed them canned pumpkin. Not pie filling mix, but pure pumpkin. It works really good and they love it.
Many times its not a lack of water, but a lack of fiber/long stemmed forage. Can you try adding in soaked beet pulp too?
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.