This interests me a lot actually. I have an unregistered 1.0 code and I am waiting to enter it for quite some time. However when I logged in to my account management and went to "select service" and choose FFXIV it still says for me "Code registration currently unavailable.". Any thoughts? I'll wait for a while and then I'll petition SE again I guess.
There is no registration in SE account manager between the 31st of july and Open Beta. Any application for Beta must have been done before the 22nd and followed by the registration on SE account manager upon the reception of the registration code or once the Open Beta starts. If you did not registred by the 31st, you'll need to wait for P4 to apply and register.
Thus, if you have an old registration code, this will not work to add the game until you apply and register again once P4 starts.