Here's what I understand so far:
As long as you have a Japanese account or made one at one time (your region must be set to Japan), you should be able to pre-reg without any problem from your PC, phone, tablet, etc. You should get a message that says they will email you.
Then you wait along with everyone else.
One thing I'm not understanding right now is how the game will be available exactly.
I'm going to assume that for iPhone/iPad, all you'll need is a Japanese iTunes account to get it, but for Android you might have to resort to some trickery to get it unregion locked if it appears region locked in the Play store.