The OLC memo.
[...]
MEMORANDUM OPINION FOR THE COUNSEL TO THE PRESIDENT
On December 17, 2011, the Senate agreed by unanimous consent to Âadjourn and convene for pro forma sessions only, with no business conducted, every Tuesday and Friday between that date and January 23, 2012. 157 Cong. Rec. S8783 (daily ed. Dec. 17, 2011). During that period, on January 3, 2012, the Senate convened one such pro forma session to begin the second session of the 112th Congress and adjourned less than a minute later under its prior agreement. 158 Cong. Rec. S1 (daily ed. Jan. 3, 2012); see also U.S. Const. amend. XX, § 2. You asked whether the President has authority under the Recess Appointments Clause, U.S. Const. art. II, § 2, cl. 3, to make recess appointments during the period between January 3 and January 23 notwithstanding the convening of periodic pro forma sessions. We advised you that he does. This opinion memorializes and elaborates on that advice.
[...]
Looks air-tight to me. Of course, that won't prevent someone from filing a lawsuit.
Cheers,
Scott.