Hmm, not so easy.
If the room is never visible from "outside" the room, you could
maybe build that room with a low-brightness static light to generate
the shadows well.
Then whenever you are in that room, set the camera.ambient to low when
the light is OFF, thereby hiding the room contents and the shadows.
When the light is ON, set the camera.ambient to high thereby revealing
the room contents and the shadows.