I don't have any definite answer here, and it may vary from hotel to hotel or even depending on the specific personnel involved. But in my limited experience, if the company or US agency for whom you're working can book the rate for you, that gets you the USG rate. Or you can get a letter from the agency to the effect that you're a contractor. I don't know if your showing the hotel a contract that indicates you're on USG business would do the trick.