What if we extend the discussion to non-contact fouls?

A1, running with a live ball, steps out of bounds at B's 35, but doesn't know it. He proceeds down the field, turns around at B's 5, and enters the end zone demonstratively backing up, Johnny Rodgers style; an official who also doesn't realize the ball was dead flags it for taunting. Is that a form of USC that can be assessed if the violator thinks the ball is live (or regardless of what he thinks or whether the ball is live), or is it a foul only if the ball is actually in play?

I've only got one season of football under my belt, but it seems to me taunting is taunting.

I concur with Adam. Unsportsmanlike Conduct doesn't care what the status of the ball is. It can occur by any player, coach or other team representative at any time during the contest.
On the other hand, if the wing official trailing the play knows the player stepped out of bounds and let the runner get 30 yards down field without his partner knowing the play had ended, there are communications issues that must be addressed between the crew.
If it turns out the whistles had been blowing, the crew all knew the play was dead and the defense had already started to return to their huddle, then I would rather assess 5 yards for DoG, instead of 15 for USC.

This isn't an extension, its a whole new topic.

USC can be either live-ball or dead-ball, and what the "violator thinks" about the status of the ball is irrelevant. The basic spot is the succeeding spot. 10-4-5a

Is it always irrelevant? If the player with the ball thinks it's alive but the opponents he's nyah-nyahing to know it's dead, that doesn't seem like much of a taunt. Of course USC can occur regardless of whether the ball is alive, but couldn't the status of the ball affect your judgment as to whether an act was unsportsmanlike?

In this case ISTR having read of either a rule or a ruling in either Fed or NCAA that identified certain tactically useless and demonstrative acts while carrying the ball over the goal line -- backing up or diving while out in the open -- as specific cases of USC. So my question is whether those are limited to live ball situations.

My own assessment is that the essence of unsportsmanlike conduct lies in the intentions of the person doing it, so that it shouldn't matter whether the ball is alive or not, but I wanted to know whether the rules comport with that concept.

FYI, Robert: Footballs are not "alive" -- thank goodness.

NFHS rules do.

You haven't seen the way some of them bounce, then. Pat McInally's I think weren't just alive, but trained.