Hot bran mashes are something I do...but it is like anything else, moderation is necessary.
It is an old school thing and it makes me feel good to give them a nice warm sweet treat after a tough time. It is a little empathy on my part as I remember the how I felt down there after having children and the thought of going to the bathroom afterwards was anything but mind settling. So I do give them a bran mash just to get a little extra fiber in them, and at the same time they get a little sugar from the molasses to give them a bit of an energy boost, and some nice warm water to hydrate.
I do not give a large amount, maybe a 1/2 to 3/4 cup bran with a fair amount of molasses and lots of water so it is the consistency of runny oatmeal. I buy the box on the cereal aisle at the grocery store made by Quaker if that gives you any idea how much and how often I feed it. Usually a box lasts me a year.
It certainly is not a necessary thing. I do pretty much give my mares free choice hay after they foal and I allow them to be picky, if they only want to eat the premium and use the rest as bedding, so be it.
Good luck and make sure and let us know when she foals!