"Rules" in this area are a bit fuzzy. Receptacles are "not permitted" in areas like the space above a suspended or drop ceiling, and yet, there are devices (condensate pumps etc) that are plug-in as OE, and they sometimes have to go into these spaces.
To answer the OP's question literally and directly, though, it is no code violation to put ANYTHING in an "unoccupied" space. As soon as the last person leaves a room... it becomes an "unoccupied space".
