General Discussion

is illegal. Short of that, some writing or similar is not considered to be covered under the law.

And while a small stamped verbiage is likely not going to be problematic, I wouldn't push it with a large stamped area that obscures the major legal aspects of the design--including the Secretary of the Treasurer's signature, most definitely NOT the serial numbers (big, big, big no no) or similar. Trump's signature should be fair game.