Grab some non-molasses beet pulp from the feed store and soak about 1/4 to 1/2 cup of it, then put in supplements. The bag will last you forever, and that little amount of beet pulp shouldn't make her gain weight. You can also use hay pellets or soaked hay cubes, Enrich 32 (I think it is..) which is an RB designed to be lo cal, or even some kind of generic "complete feed" from the feed store. Pretty much whatever you choose that isn't oat based or containing large amount of flax, rice bran, molasses, or grains other than oats should be not too high in calories or NSCs. And you don't need to feed a lot, just wet a bit of whatever pellet you are feeding and mix in the supplements.
They say money doesn't buy happiness -- well happiness doesn't buy horses!