The "Manufactured housing" industry has argued strenuously that the controlled factory environment, quality control measures, and extensive engineering involved all make a good case for allowing them to do things that would otherwise not be allowed. This argument does haqve some merit, and has been successfully used many times.
Moreover, the products are typically listed as assemblies- without there being any listing of individual components.
I would frown upon the use of these parts elsewhere- there is a lot more to consider than "does it work today?"
Rather than simply saying "no sticker, no approval," I would hope that an inspector would take the time to compare the item to listed components, review the various product standards, and consider the qualifications of the parties involved.
It really does seem unfair for ABC Mfg. to be allowed to do something that Joe Tradesman cannot. We also have a culture that rewards initiative, ingeninuity, and expects individual responsibility. This is the antithesis of the "nanny state," where everything needs approvals from sundry parties.
If it isn't specifically forbidden, it is allowed- in my book.
