I am not sure. I haven't done the "Tour of the Castle" in a long time, but I remember for the TotC you go up a separate set of stairs with a different hall of portraits integrated into this staircase. I don't think the FastPass would go through here, but then again, I am assuming this since we've never seen the FastPass queue open. Maybe this was the intended FP queue and it evolved into the Tour queue? Who knows...