"The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any subject relating to the Duties of their respective Offices, and he shall have power to Grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment."Nothing about "the process has to be complete".
I agree that the President can do this. I think the hitch comes in with the "Offenses against the United States". Naturally, Libby's defense was that he committed no offenses, so a pardon before the verdict would have looked bad, both for the President and Libby. Now that he's convicted (erroneously in my opinion), the President should pardon him.