Title says it, I don't see why this would be an issue. It does 2 less pierce damage than a heavy lance, and is about as long as a practice lance or red tassel/war spear. It would be more for fun than usefulness, as you probably wouldn't be riding anything more than say a rouncey while using this on horseback.
Just a thought. I've got a little crush on this weapon at the moment.
EDIT: I did forget about the distance change from holding a weapon in the middle as opposed to the end. but you could hold the flamberge from the upper grip, like you do when thrusting with it normally, just with 1 hand instead.