I've seen American riders flying with Bromptons in the overhead lockers.
Do Americans have different dimensions for luggage than the UK ?
They do have friendlier airlines I'd think. Personally here in Europe I'd not dare trying to get the Brompton on board of a plane (apart from the question if it would be a good idea anyway) and the space available in the cabin prohibits it anyway.
On the other hand: When digging around the net for information before buying my first Brompton ages ago I stumbled over the webpage of a flight courier whose job it was to transport small items like important documents worldwide and on short notice using normal flights. He very much enjoyed his job as he saw the world - with the downside of having to travel with only cabin luggage due to more speedy on- and offboarding. One day he discovered the Brompton and this was a gamechanger for him as he said. He took it with him on all of his flights and all of a sudden he could discover the towns he visited easily in the very limited time he had available. It must have been in the mid 2000s when I found his story and even back then his homepage looked pretty oldfashioned - so I guess the story probably happened in the 90ies.
Clearly things changed to the worse with cheap airlines, stricter lugguage restrictions and airport security becoming much stricter after 9/11/2001.
If you search for relatively recent stories bromptonmafia are a good starting point as is the hashtag #bromptononboard. It seems to be mainly a US domestic affair and Southwest being the airline most open to it. Twitter and instagram show a lot of pics in that regard.