rice bran is definitely good many horses love it some don't 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 don't over feed it. I can't 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 don't have to feed very much of it.
Good luck! I feel you pain in the easy keeper department :p
My mare= My life <3