Explicity banned.
I do not recall seeing such a statement, but neither do I see where explicity permitted.
The only reference I am aware of - of metal raceways being a conductor is for Equipment Grounding Conductors (EGC), 250.118.
Along the above thought; 250.24(A)(5) does not permit the Grounded Conductor to be re-grounded after the Service Disconnecting means, but does have exceptions. I do not think most installations where the raceway would be used as the neutral could be free of being grounded somewhere.
I do not see metal raceways as conductors in Article 310, Conductors for General Wiring.
