Re: strapping
300.11 (B) is where is says that you cannot use an electrical raceway to support anything else.
It may be in the mechanical code where you will find your answer regarding ductwork, and in the plumbing code where you find your answer regarding pipes.
In practice, we never use a duct as a means of support except where a device is mounted to the duct to serve a purpose directly related to that duct ( for example a duct installed smoke detector.)