I used to know a horse that backed great on the ground but wouldn't under saddle. He would rear, buck you name it he did it.
I would suggest the using another person way. Have the person on the ground give the cue while the rider gives the cue, then try to slowly wean the person on the ground away.
Or wait it out. I had a horse once that would not back at all. So I sat there for probably 20 minutes with even pressure, till the horse lost balance and took a step back, it was downhill from there. Its you just got to start and be ready to reward, as every one else said, the smallest possible try.