How about giving the picker a “src” attribute that takes the URL of a document in a prescribed format. If the attribute is omitted or invalid, or the document is malformed, fall back to an internal default list for the browser.

This would allow the UI to remain the same (with searching, sorting or whatever), but let the page author use an alternative list of values. Governments or other bodies could set up “authoritative” lists, but page authors can still use historical lists, shortened lists (“countries we ship to”), or extended lists which are hosted at a more local URL.