Ah... oh, I think I know what happened now. After playing through the game the first time I used the new game option on the main menu, but ultimately moved over to a save file at the first point the story can diverge. Creating a new game would have removed the button, and since I didn't meet Leo before switching to the saved game, I never got the option back. Gonna go check and see...
(Edit) That fixed it, alright. Thanks.