ISO 3166-2

[Crissov]

Unicode Emoji 5.0 is now in beta stage, due to be released with Unicode 10.0 in June 2017. It defines Tag sequences for subregion flags.

// pseudo code
string function subregionEmojiFlag(string code /*example: "GB-SCT" = Scotland */)
  var emoji = U(0x1F3F4); /* Waving Black Flag */
  var (region, subregion) = code.toLower().split("-");
  for each (codepoint in region+subregion)
    emoji += U(codepoint.lowercase & 0xE0000);
  emoji += U(0xE007F); /* ETS terminator Tag Cancel */
  return emoji;
}

You may as well just support it right now, since it is – sadly – very unlikely there will be further changes. (A previous proposal used the Waving White Flag U+1F3F3 instead, like the Rainbow Flag ZWJ Sequence does, but it requires U+FE0F Variation Selector 16 to be rendered as an emoji reliably.)