Subtitute "her ID was leaked by a person who is under the law" for "Plame was not covert" as being what you refer to as "the threshold element."
What you describe is apt to be the primary line of defense by Libby, but it will be as tough to prevail in taht as it was for Miller and Cooper to get out of testifying ON EXACTLY THE SAME GROUNDS.
Miller and Cooper asserted that they did not have to testify, because Plame was not covert, ergo, there is no crime, ergo, there is no need for their testimony.
Miller went to jail over the issue.
Does the mere revelation of her "identity" have any legal meaning?
Is it wrongful for "a person under the law" (by which I assume you mean a person bound to handle certain information within certain restrictions) to reveal the mere identity of person when that identity has no particular legal protection?
If I'm a "person under the law" because I, say, have a security clearance, and I reveal information that in fact is not prohibited to reveal (by anyone), I've still done something illegal?