I can't be sure but I don't think it would work like that. I assume the character information all comes as a packaged bundle and it's still going to be sending the data about tail shape even if it concludes with "tail: 0".
Printable View
No, the data would still be there, because it's a setting that must be set in the character creator. If you don't pick one, it's just left at the default. It can't be set to "nothing".
For comparison, NPCs that use custom faces or heads still have underlying settings applied that don't match what you see.
All the Scions have custom faces even though they seem to use standard NPC designs, I think to add their neck tattoos, and if you look up their data you'll see the apparent settings don't match the actual appearance. It's just there because something needs to be there even if it's overwritten by the custom appearance.
For example, this is G'raha Tia's data - from actually looking at the character, he's clearly using face #3 (or really a custom face based on it) but his data shows face #1. That data isn't actually getting used at all to display the character, but it's still there.
Meanwhile, the Garlean characters are generally either Elezen or Hyur NPCs with custom heads. Nero, for example, is an Elezen-based Garlean and if you look at his data there's not only unused face data but also a setting for how long his Elezen ears should be. That is never ever going to be relevant, but it exists because the character template needs something in that box.
I very much agree that rather than hiding tails, gear should be made to accommodate them instead. Imo they got lazy (or rushed) in that department since some ARR and HW gear did have tail holes. The lv 50 Noct set comes to mind, where the chest piece actually has a tail hole, but funnily enough, my tail (as a male Miqo at the time) clipped right next to it...