Perhaps a lawsuit for fraud could be made against the United States for using the word "dollar" on its notes, when the dollar is defined in federal law as a given weight of gold, and U.S. Notes by law cannot be redeemed for gold?

You would have to ask the Secretary to give you "dollars" i.e. gold corresponding with the amount of United States "dollars" on the bills you present, and sue for fraud if they refuse to give you "dollars".

The Federal Reserve seems to define a "dollar" as nothing more than a dollar U.S. Note. But the U.S. defines a dollar even today as a given weight of gold.