I guess that depends on to what extent you buy the argument that Leo felt enabled to crown Charlemagne because of a perception that the imperial title in Constantinople, being held by a woman (and a controversial one at that), was "vacant."
My feeling is that Irene had less to do with it than the retreat of the Byzantines from their position as suzerains of Rome. The emperors had traditionally protected the papacy, or at least pretended to, but it was now apparent that the dominant force in the west and the only one capable of extending that protection to Rome was the King of the Franks. If that's true, Irene's usurpation or lack thereof probably wouldn't have an impact on Leo's decision.