Coming from somebody who has always wanted to visit Tokyo, I can only imagine what the HN meetups are like.
We run a casual, low-pressure mixer kind of event about once a month. If you come to Japan, you can follow our blog (http://www.makeleaps.jp/blog/en/) or signup on Doorkeeper to get notified of events. We get lots of visitors from abroad swinging through and we're always looking to help support other startups in Japan or small businesses trying to enter the market here. It's a really good crowd of people.
There's also a meetup for HN readers in the kansai area of Japan if you're visiting there.