When can a guild be cloned based on Olympia's B stage 3?
Rules (page 9) say:


  for the third stage, the choice of the Guild is made at the end of the game, when counting points. The player totals the victory points as they had built that Guild.


Since Courtesan's guild gives no victory points, selecting it will be no use for the player. Note that, Olympia B was designed at a time when all guilds were effective only during scoring.

See here for more discussion.