I would definitely take Dubdub's advice and go to the mobile shop in the Exchange as the folks working there will know how to work through the all of the contract mumbo jumbo. All of the newer UMTS devices have the correct band (UMTS 2100) for Japanese and Korean 3.5G service, and DoCoMo is the only carrier that has ever sold BlackBerry service in Japan.
Here is their page for the 9780, but you can use any of the newer UMTS ones like the 9810, 9900, or 9860. They simply don't customise all of the models for the Japanese language: BlackBerry® BoldiTMj 9780 | »i | NTThR
Hope this helps, thanks for your service, and enjoy Japan!