As of March 2008 they meet on Saturday at 3:00pm directly east of NHK Hall.

There's a open concrete park area off the North East corner of NHK Hall, just south across the street from Yoyogi Park where it looks like critical mass should meet. Maybe one day when the movement grows larger in Tokyo. But you're about 75 meters too far north. Head south down the tree lined pedestrian boulevard with all the buskers and they'll be on your right just about here.

Pics one of these days.