The dress code shall be national dress...
This clause is certainly interesting, in the US case.
The US geography and culture varies WIDELY. If I were to have to specify a "National Dress", I would say that an urban "hip hop" dress is the most standardized across the US, at least in the larger cities.
Therefore, I would say the US "National Dress" would be pants that are too long, and WAY too big, worn WAY below your waistline and held up with a large belt. Shirt would be a short sleeve t-shirt of some sort, with a lot of tatoos. Definitely an oversized "baseball" cap, with lots of bling on it, worn so that the bill is at some angle off the to the side. A good amount of blingy jewelry, along with probably some metal attached to your face, and large, "goofy" looking sneakers, unlaced of course.
To me, wearing this would be a legitimate US "National Dress". I don't think Qatar would approve....