Yes, it is a loophole, but just in one law, not in the constitution. Congress delegated to the Treasury the right to mint coins of any denomination, even if the intent was different.

I must admit that the separation of powers issue had not quite occurred to me, since the entire scheme is self-defeating and would scare holders of the US debt anyway. People would be (and are right now!) worrying a lot more about a credit rating downgrade or a default than a separation of powers concern.