IMHO, the only possible way that I, or my BJ for that matter, would have ever flagged such an act would be if the player(s) in question also turned to face the opposition that was trailing him while he was ardently displaying the "
numero uno" digit toward the heavens. By "the only possible way", I'd have to see the intent of the perpetrator as proof positive that he was indeed taunting. I have been told by some of my college brethren that their commissioners pretty much dictated that if inside the 3 yard line, do not flag them but to dole them out a rather stern "BUTT-chewing" for what they did. I furthermore understand that TASO subscribed to this very same edict at this years State Meeting in San Antonio.
This is much the same way as some player "flipping" or "somersaulting" himself into the end zone. If he is way ahead of the defenders and does it, then that can only be construed as "showboating." Conversely, if he is encountering any defensive player(s) that are either in hot pursuit or defending in the vicinity of the goalline, and then he "flips," then in my minds eye, that act would be totally acceptable!