Here's what I read in this item ("Pelléas" de Debussy au musée d'Orsay [Debussy's Pelléas at the Musée d'Orsay], May 26) from France 2:
The Parisian museum is producing Debussy's opera, which will be performed from June 1 to 6. Pelléas et Mélisande will be presented in its original version, with piano accompaniment only. The performances will showcase bariton François Leroux, with tenor Jean Fischer (Pelléas), soprano Ingrid Perruche (Mélisande), bass Renaud Delaigues (Arkel), and mezzo soprano Marie-Thérèse Keller (Geneviève). Two children from the Maîtrise de Notre-Dame, in alternation, will perform the role of little Yniold. The staging was created by actor Vincent Vittoz. Before the 1902 premiere of his lyric drama, in the Salle Favart in Paris, the composer had presented this version with piano for a circle of his close friends.If you're trying to picture where exactly such a performance will take place in the museum, there is an actual auditorium where this sort of concert event takes place, although I don't know how or if the opera could actually be staged. For another view on the event, see Jacqueline Thuilleux, A l'Auditorium du Musée d’Orsay, «Pelléas et Mélisande» (in Figaroscope, May 19), which also informs us that Alexandre Tharaud will be at the piano. There is also some information on this production available from the Musée d'Orsay.