Contact Bill Valavanis... I don't know if he goes every year... but I know he takes groups. And from what I hear it's a wonderful way to go about it, because he has so many connections over there you get to visit a lot of the best gardens.
But its the Green Club you want to go to... I've seen photos only, but I can't imagine all of that bonsai - pots - stands - slabs - etc in a multi-story building for sale... makes me shiver just thinking about it. lol
[In 2011 and 2012 the show will be moving to the Tokyo Metropolitan Industrial Trade Center, located near the Asakusa Kannon Temple in Asakusa less than a mile to the east of Ueno while the museum is remodeled/rebuilt.]