2011 422.16 describes cords that are field installed -- 400.8 has a charging statement of ?unless specifically permitted in 400.7? (8) is specific to an appliance that has a cord connection attached from the factory --
There are numerous AHU in the US that will have a humidifier option, in which, the appliance comes from the factory with a cord & cap connection with a listing for the application to be installed as such -- Large Hotels come to mind.
400.7 Uses Permitted
(8) Appliances where the fastening means and mechanical connections are specifically designed to permit ready removal for maintenance and repair, and the appliance is intended or identified for flexible cord connection