You're playing a secondary game, countersignalling by conspicuously assuming a lower-status pose. You're both aware that you could instead assume their pose and play chicken with them about who will submit first. By "choosing the high road," you're saying that you've noticed the status game in play, and that now their implicit status move has become an explicit status move. And humans have strong norms about the group disapproving of explicit dominance. (See: democracy.) So when the person displaying the dominant behavior realizes they've been "found out", they stop.
Also,
> The feet on the table one, for example. To what extent is that signaling high-status simply because it's a very rude thing to do?
To answer this, consider the converse: to what extent could the fact that it's a rude thing to do be explained by anything other than it being a show of dominance?
In the case of picking one's nose, it can be considered rude because it is a private grooming behavior considered to be not especially hygenic by modern standards. It exposes bits of one's internal "grossness" to others. It would be uncouth even without dominance circuitry.
In the case of putting one's feet up on a table, though, I don't much see what could be wrong with it other than that it's a show of dominance. Perhaps we might be slightly wired to predict that feet are smelly or covered in dirt, but in the modern white-collar work environment this isn't the case--so it shouldn't be rude. And yet it is. So it's a dominance behavior.
(And in the case of scratching one's arse, I'm not quite sure--that doesn't seem dominant to me, but nor does it seem rude. It just seems like an artificial restriction created by some Tough environments[1] to stratify people by "those who can ignore all their bodily urges and pretend they don't exist for longest." This is called etiquette, and is a whole different kind of mating-fitness-signalling--more like birdsong than apes beating one-another up.)