Turns out you can find Lore in the most unlikely places. Browsing General Discussion at random, I noticed something in a link to an old piece of concept art.

Relevant detail:


Apparently, that "eye" tattoo is called 「シャーレヤンの印」 (the "Crest of Sharlayan"), at least internally. Doesn't really answer much on its own, if we can even consider anything on the above canon.

If we can... I guess we can see it as a patriotism thing? We don't see it on people like Louisoix or Matoya, who deliberately (and bitterly) cut ties with the motherland, but we do see it on the Scions who once left with Louisoix but sorted things out with the motherland between 1.0 and 2.0. Moen and G'raha are (were? ) in Eorzea for presumably the first time, but each seem to have done their own bit of traveling and probably wanted it as a reminder of where they came from. We also don't see it on Ala Mhigo-born Minfilia, because she's a foreigner.

Just leaving this here for reference in case it comes up again, or we start seeing it on actual disposable guest characters before too much longer.