On the one hand I don't think they can just add the option (since they don't seem able to update character customisation at all) *BUT* I think it's actually already connected to individual costumes rather than the character settings. It's not used much, but there's the Evenstar Coat - whether you appreciate its approach or not, it's clearly deactivating the normal tail with the replacement built into the outfit.

There's also the Exarch, who programming-wise is a standard Miqo'te NPC but doesn't have a visible tail, so I would guess that his robes have the same effect of making it not display. And Yugiri in her earliest masked appearance is also a Miqo'te with the tail replaced by an Auri tail, so perhaps that is achieved through the same method?

If I'm speculating correctly and they can indeed link tail visibility to costumes, maybe they just need to do more of that where it's appropriate.