maintenanceguy,
As you noted, SO cord is one of the types of cord listed in article 400, not one of the wiring methods listed in the 300's.
However I see no reason that any given cord could not be multiply rated as both one of the types listed in article 400 as well as one or more of the methods listed in the 300s, in which case that cable assembly could be used for fixed wiring exactly as described.
I don't know if any such cables are actually manufactured, nor if there are specific requirements that make such a dual rating impossible, for example permissible wire stranding. I can imagine that there are situations where a single cable, dual rated as types TC and SO would be quite useful, essentially permitting permanently installed extension cords (with suitable protection and support, of course).
-Jon
