I noticed that the US Flag Code calls for all in attendance to place their hand over their heart, making no mention of the origin of the people. I would assume then, that means even non-US citizens would be expected to do so... For example, tourists from another country at an American baseball game... Is that a correct assumption? If so, would that also apply to non-US dignitaries in attendance?

When I was in Japan, I would always stand and face the Japanese flag during the playing of the Kimigayo, but I would not sing with those present, as I didn't feel it would be appropriate. Plus, there was (and still is) a LOT of political contention about playing the anthem at official school events. The two sides are very polarized and I didn't wish to make a political statement one way or the other.