Agreed, the United States participation in the United Nations is legal by an Act of Congress.
As for a Government Official holding the pro tempore title bequeathed by the United Nations, I do not think that is a legal violation of the
Constitution.
The reason being is that this has already been covered in U.S. Code Title 22, ch7, subch XVI, 287. Representation in an Organization, Item F.
Nothing contained in this section shall preclude the President or the Secretary of State, at the direction of the President, from representing the
United States at any meeting or session of any organ or agency of the United Nations.
U.S. Code Title 22c7xvi287. Representation in an Organization
Neither the President nor the Secretary of State need Congressional approval in the appointing of themselves as an ex officio as representative of the
United States in any organ, commission, or other body of the United Nations.
[edit on 30-9-2009 by fraterormus]