rice bran is definitely good many horses love it some dont though, my horse LOVES is with warm water added. you can test things like that out with your horse if you decide to feed it, just make sure you dont over feed it. i cant remember the max that is okay to feed--hopefully someone else does!
you can try other fat supplements as well. i really like cool calories 100, it has 5x the fat of rice bran and you dont have to feed very much of it.
good luck! i feel you pain in the easy keeper department :p
My mare= My life <3