Excuse me for being slightly late...
Call or e-mail one of the local cab companies and book a "Wagon Taxi" with more luggage room. The fare is 400 yen plus the usual . You can reserve in advance at particular time and place. You have to pay some extra hundreds of yen for the booking.The fee is called "Geisha Ryokin" It may sound interesting for gaijin FTers out there

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Following is a cab company which IOC hired cars from for the Nagano Olympics.
http://www.tokyomk.com/eng/index.html
http://www.tokyomk.com/jpn/img/ryoukinnew.jpg