ozstamps etc.
Good idea about not getting past gate agents but from my experience they are usually too busy to notice, especially processing people at the last minute - time constraints, etc.
I have yet to see a gate agent peak over the podium and look at people's carry-ons for size, and even if they did notice, they don't have time for dialogue and the objection that the passenger is going to give them at that moment. Sometimes there is only one gate agent with other people on line so it would be difficult if not impossible.
I say build a hatch inside the cabin with a chute down into a cargo pit and write your own bagtags to fill out the city of destination.
Many times people are not even wanting to relinquish the bag at the end of the jetway to be taken by a rampserviceman to be put in the cargo hold or they are well within the aircraft wasting more time moving back to the jetway to do that.