Any US building codes about a door opening into a hallway near a stairway?
There's no such thing as US building codes, there are state, county, and municipal codes which vary widely between areas. The only way to know is to ask, which is what I'd recommend you do. There's no harm in it, simply call the office that is responsible and ask. You don't have to say you've already done it if you're worried about it, you could say you are planning on doing some remodeling and ask them if a door opens into a landing if that's OK and see what they say. Worst case is you have to turn it back in the way it was.