If worse comes to worse, stay polite and sweet-talk the agent into a grabbing a gate-check, festoon your bag with crew tags, and then on arrival run down the jetway stairs to get it at your arrival city before they toss it over to baggage claim. I do this whenever I board last and there is no space in the cabin, which is more often than not anyway if I've got the whole bag-train behind me.

Thanks for the heads up.