I would be fine with that, I would also be fine with an item dropping from a raid or dungeon that's needed to craft an item and that thing being untradeable.
If the item drops from new-coil (T6-9) we effectively have a one-week-lockout item (provided you can even go T6 right off, not everyone will). It could also be bought with Soldiery tomes and I'd be fine with it. And make it untradable. Possibly crafted in something also untradable, at i100. Melding it would no doubt cost millions if its anything like melding Vanya is (100k+ materias with 30% chance for your 3rd slot, 25% for the 4th and 15% for the 5th - of course you also pay the fails).