I found the question interesting, for the use cases could really bring practicality but I think it is impracticable through a specification and unlikely that developers of browsers implement.

An Open Source library with an active community and up-to-date (or historical) data in a public JSON would be exceptionally more useful. A JavaScript (or other) language could easily handle groups, allowed or blocked listing, or even a custom entry that could display country flags.

I particularly use ISO 3166-1 alpha-2 on “select” elements since this ISO is usually the default internet since this is the default used in DNS (ccTLD)

The reason for not finding Scotland in ISO 3166 and its derivations is given to the fact of reservation:

https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2#Exceptional_reservations