His name is Koji Attwood and he's tremendous. I hope he starts getting better known; he deserves it. He's a great transcriber too, he did transcriptions of Schubert's Death and the Maiden and Tarrega's Recuerdos de la Alhambra which are fabulous.
Actually, Soheil Nasseri was the pianist onstage in Acts 1 and 2 and also for all of act 3 from the orchestra pit. Koji Attwood played Acts 1 and 2 from the pit.