It has been amusing reading the responses to my OP.

I wrote to US CS today (as it was a USX flight) asking for their specific policy with regard to such situations. I did not ask for, nor do I expect any compensation. What I do want to know is what each airline's policy is, so that, if and when the situation arises again, I will know exactly what I ought to do.

Secondarily, if a particular airline gives me some sort of fuzzy, non-committal, "unwritten rule" type of response, I'll be less inclined to patronize that airline.

In my experience POS (or COS, or whatever the appropriate term may be) are usually uncomfortable knowing they are causing others discomfort. A known policy, such as at WN, is helpful to all - and reduces the possibility of a thoroughly avoidable confrontation.