Can anyone offer a quick primer on this topic? Is it enough (or even worth the effort) to go to the gate of "Air X" with a ticket on "Air Y" and ask if Air X will accept the ticket. If yes, do you need to get Air Y to endorse the ticket first? What incentive/disincentive does Air Y have to do this?

Does the fare basis booked on Air Y matter much? I would guess that if Air X would rather fill an empty seat, even at a significant discount, rather than allow a lost revenue mile.

I would be willing to approach Air X, for example, if my discount Air Y ticket is one-stop, but Air X is a non-stop.

Any thoughts? Any rules or restrictions? Is there an accepted procedure?