I tried emailing support yesterday. I finally got a response a few minutes ago:

Thank you for contacting the Square Enix Support Center

We apologize for any inconvenience in this issue. The error with the region mismatching has been resolved. Please try your code again on the registration page.

http://preorder.na.finalfantasyxiv.com/

Thanks,

Square Enix Support Center
Guess What? I'm still provided the same stupid code that still doesn't work.