Revelations from Encyclopaedia Eorzea III

[Iscah]

I think the proto-alphabet has actually been used for the Loporrits (their "On Air" sign from the tribal quests, the 'n' is visibly the proto-n and not the Eorzean-n) and the Twelve alliance raids as well.

That would make sense. I'll have to double-check against the Twelve's monument later.

Still, that makes the choice of making it a mere prediction (later shown to be improbably accurate) extra odd, because it would be very straightforward to attribute the third alphabet to having been observed in text of Loporrit origin.