I didn't know Immodium was okay for horses? That would be handy to have on hand in the vet kit for me in the future.
When we had a foal with diarrhea, the vet said give 10cc of Pepto-Bismol 2x a day and that seemed to do the trick fast (only gave it twice total).
What about Probios? That might help her, too. They sell them at TSC and they have an expiration. I think there's a lot more "active" stuff in the tubes than when people try and give yogurt for the same purpose.
Also, would giving the dam Equidone help her produce more milk? (I don't know, am asking in case it helps -- I know it stimulates milk production but don't know it can increase it when it's already being produced.)
When our dog had diarrhea a year or so ago, the vets gave them something that I think was the dog version of biosponge. It worked well.
Good luck!
PS note about immodium in dogs -- it's okay for most but not for some herding breed dogs. For some herding dogs, it can be toxic (sort of like ivermectin can for some herding dogs).