It has to be in a metal wiring method. More important is what do you plan to plug in to that receptacle. You can only have equipment "necessary for their direct action upon, or sensing of, the contained air" (300.22(B))and I would want to see the listing. A cord is probably not going to be allowed unless it is plenum rated.
I'd think it would be fine .. FAU's up in attic spaces require a power source which are not meant to be use by any occupant .. as long as the power source is readily accessible for maintenance or service and conform to other necessary NEC guidelines as far as approved boxes,fittings etc .. really It just can't be buried or covered up.
Anyone claiming to know everything about Electrical, is wrong.
"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".