Unfortunately i didn't get a screenshot, and it must have been a glitch, but I got onto my Imperial Horse and my staff stayed on! ZOS said that if they did it the way they have weapons on the characters back right now, it would clip into the horse. But when it happened to me, it was fine. No clipping, and it didnt look like the staff was any higher on my back than normal.
I dont know how it happened, but ZOS it can be done