All the Japanese orienteers I know are in Kansai, but I looked up the websites for you.
I found events on the following days that might be of interest to you.
I've listed the ones in the Kantou region. If you're interested in any, post here, or email me. I can contact the people involved, if you like.
Sept 30: Tokyo Institute of Technology Cup (Saitama)
Sept 30: Kanagawa Park/Sprint Event (Yokohama) (I take it you?ll be interested in this one, so the details are below)
Oct 14: Kantou Student Orienteering Association Beginners' Champs (Sanmu-city, Chiba) (but anyone can participate)
Kanagawa Park/Sprint Event
Very accessible by train:
http://maps.google.co.jp/maps?f=q&hl=ja&geocode=&t...
Uses E-Cards, 1:5000, 2m contours
Enter-on-day possible.
Cancelled in the event of torrential rain
Registration is at the 海の公園管理センター (Sea Park Management Centre; it might be referred to as Umi no kouen Management Centre)
Start is from 10AM. Registration is 10-11:30
Enter-on-the-day cost 1000 yen. This includes e-card rental. You can't use your own e-card even if you have one.
Can not wear 'spike-shoes'
Access:
Train: 'Seaside line' Alight at Hakkeijima Station. Use Hyperdia (
http://www.hyperdia.com/cgi-english/hyperWeb.cgi ), and type in Hakkeijima as the destination.
They say there is 'guidance' from the station. Presumably this means streamers or signs. 2 min walk.
There is another train line they suggest, but this involves a 25 min walk.
For enquiries contact nobu.chika@nifty.com ("Imai-san")
Between 7/27-8/31, contact dzc03145◆nifty.com, replacing the diamond with an @. (I can't provide a name here, because the characters could've been read in a number of ways)
I recommend that you contact them first (in English), because then they will expect to be seeing you. Also, they'll probably be very welcoming to you if you contact them first. They will probably have at least somebody in the club who is willing to show off their English skills.