This would be a 300.22(C) installation. It looks like 300.22(C)(2) would permit this.
300.22(C) doesn't apply here, 320.22(B) does. As soon as you tack that pan up, the joist space becomes a duct. The ONLY boxes allowed in ducts are those required by function to be there- smoke sensors, dust sensors, etc. Only cables connecting to these boxes are permitted by 320.22(B).
