Previously, Japanese prepaid and rental phones didnt allow Internet access. This year, that has changed.
The model 707SCII allows Internet access to foreign visitors but ONLY if you do a rental and give them your credit card. Basically, you use it like a regular citizen, and they just charge you for the service when you return the phone. You must pick up and return the phone at the airport! So, if you leave Narita Airport without getting the phone you cant do this.
Anyway, you can either reserve the phone online or just get it in person from any Softbank stand at the airport. There are Softbank stands in both terminals at Narita.
Once you have this, you can visit sf4.jp just like a regular Japanese cell phone and register a card. They sell the cards from most arcades for 500 yen. If you need help navigating the moonspeak on your phone, just ask an arcade attendant, theyll help you.
This was mostly written for the handful of folks about to visit Nippon for SBO, but thought it might be useful for those intending to visit later as well!