Hi, Alice,
I've searched for forums open to the public for a few hours now, but couldn't find anything specifically for spouses or family. Most sites open to the public that I did find were personal blogs that offer information to transgender individuals themselves. While one non-public site looks like it just might possibly contain something, I'd have to become a member to find out—and to do that I would need an invitation or a referral.
This being the case, at the moment the Okayama University connection I mentioned earlier looks the most promising. The university hosts the Japan GID Association, and consequently is likely to have the most information on additional resources.
For what it's worth, membership in the Japan GID Association is available to the parties concerned and their families. The annual membership fee is two or three thousand yen. In lieu of a link, please try googling:
"GID (性同一性障害) 学会"
—the relevant page should be on the top.
I wish I had more to offer, but since I myself don't belong to any relevant group or organization I'd have do some more sleuthing, and most likely make a personal visit to assess whether what I may find would be safe to recommend.
Come to think of it, it does seem a bit curious that there seems to be so little interfamily association. On the other hand, people in Japan don't rely on counseling or group support at all in the same way they do in the West. The society is much more oriented toward the family on one hand and community as a whole on the other.
That's perhaps why e.g. even the Okayama family group (not the association) is headed by an individual that is receiving treatment at the university, rather than the university itself. Her personal site, by the way, can be found by googling
"ナオコの不定期日記"
Again, my apologies for not being able to find anything better.