To be sure, Kennedy also claims the president made some recess appointments at the wrong time: i.e., during short breaks within a session of Congress, which he says shouldn't count as a "recess." That argument might have some merit, but it's not relevant to any appointments made between now and January, when it's indisputable that Congress is officially out of session.
The best argument against an election-saving Rehnquist replacement is one specific to the circumstances: Just as President Clinton couldn't use the power of executive privilege to interfere with a lawsuit against him in his private capacity, so President Bush can't use the recess appointment power to interfere with litigation that affects him in his private capacity as a candidate. That makes good sense. The Framers assumed that some mechanism for correcting abuse of the recess appointment power would be possible—for example, the Senate could still vote against confirmation when it returns from recess, thereby vetoing an appointment that's not in the public interest. But, if the president were to use the appointment to decide a case that secures his own party in the White House for a second term, there's no way to correct that abuse. Lawmakers could impeach Bush if they decided the tie-breaking appointment was an abuse of power, but they can't give the office to the losing candidate.
Despite those good arguments, the president would have a not insubstantial case to make in his favor. After all, the text of the recess appointment clause doesn't impose any condition on the power it grants—beyond the existence of a "recess" and a legitimate "vacancy." That means that if the scenario laid out above came to pass, the court would be forced to decide a hard but important constitutional matter in freakishly high-stakes circumstances.
Is this all a long, long, long shot? Of course. But even so, there's room for pause. As horror director Alfred Hitchcock well knew, the most disturbing stories are not always pure make-believe—they're the ones that just might happen if luck is against us. That's what makes the above scenario so unsettling—it's implausible but not impossible.