I love Japan and the idea of urban permaculture, so this really excites me! I was planning on eventually going to Japan to get urban permaculture started, but this guy has beaten me to it. I'm a bit jealous, but I can't say I'm disappointed. I will definitely establish contact with this guy and bring him to this forum.
I'm really into the idea of reforming the infrastructure of human society to be more ecological, so I'm always thinking about when and where to introduce ecological concepts. If permaculture becomes popular in Japan, probably the most urbanized modern country on Earth, then it could very well segue into a huge eco-friendly movement that would affect all of Japan and set an example in that part of the world.
Kai's blog is great, he's definitely doing some awesome work over there. Looks like he wouldn't mind a hand though! I have often thought about how powerful the establishment of an strong urban permaculture movement in Tokyo would be, especially seeing as what happens in Tokyo by and large determines the popular culture of the rest of the country. Oh the possibilities
I can personally recommend Dion Workman's writing and forest garden (http://nakazora.wordpress.com/). Spent a week there in 2012, totally changed my life.
Ken Elwood's blog is also excellent (kenelwood.wordpress.com). I haven't met him in person, but by the looks of it he has a great forest garden going on, and has been doing some really neat work including starting his own sovereign micronation in Japan (http://woodlandpatchwork.wordpress.com/). He's also got a great collection of links to various people doing cool stuff in Japan.