Just another data point...

Here at AUS this morning, the F check-in agent was more surprised that I was going onward to Rio than anything else. She seemed to know about the visa waiver, and was extremely helpful and friendly, even asked if I need boarding passes (I have them on phone but always helpful to have spares). I take it they don't get too many exciting itineraries here
I did verify the bag tag that she printed AUS-JFK-GIG.