Let me ask you this. If the U.S. Constitution granted Congress the power to define natural-born citizen, would those citizenship statutes have more legal weight?
If you are following the "natural law" arguments which are the foundational basis for the birther objections, the question is as nonsensical as asking if congress had the power to redefine pi.
It is like suggesting congress has the power to define "arms" or "speech."
Congress has the power of "naturalization" which means "to make like natural." This is good enough for almost all intents and purposes, but it isn't the same thing. It is the distinction between an adoption and being actually born to a family.
The "natural law" principle argues that this characteristic is inherent, like eye or hair color, and can't be changed by appeal to earthly authorities. It is beyond their power to tamper with nature. They can only tamper with man made laws.
Maybe so, but congress does not have the power to "define" anything in the Constitution as that would be changing or amending and that can only be done by Constitutional amendment.
We have never proposed an amendment to define NBC status although I do think in an article V convention that very issue needs to be addressed.