Let me see if I have this straight. Your argument is that, because the Constitution does not define the term, it would be unconstitutional for it to be defined? By anyone? Ever?
What do you have, that indicates an ability of Congress to make any legal determination as to the meaning of the term?
Its existence as a legislative body. The Constitution is filled with terms intentionally left vague, because the Framers knew lawmakers would, you know, make laws.
I suggest spending a little more time with this topic before getting too torn up about it. I cited dicta from a well-known USSC opinion that is pertinent to the debate.
As far as terms-of-art utilized by the Founders in the Constitution, there were no definitions offered for citizen, law, press, religion, nation, state... no terms were defined.
I suppose we’re now completely adrift and at the complete mercy of whatever scheme a bunch of politicians dream up, as far as the meanings of Constitutional terms?
I want a pony. Which term should I choose to mean pony so I can get one?
It cannot be "defined", merely recognized. Like the laws of Mathematics, the meaning is a function of reality, not of preferred description.
Any attempt to force "natural citizen" to be defined by law is no different from that attempt by the legislature of Indiana to define "Pi" as 3.2.
"Natural Law" is not subject to man-made preference.