When you get to Tokyo, pick up a free copy of Metropolis (used to be called Tokyo Classified) - you can get a copy from Tower Records, or look on their website to find out other places to pick one up. When I was there a few years ago, there were one or two Kung Fu classes (I remember there was one in Roppongi) that were advertised in Metropolis.
Alternatively you can send a free classified ad to Metropolis via their website, and ask anyone in Tokyo who trains over there to e-mail you.
http://metropolis.japantoday.com/default.asp
Also people who train in Wing Chun often also train in T'ai Chi over there and there are plenty of T'ai Chi classes in Tokyo.
Finally if all else fails, you could always cross-train. I think it might be better to join a good Aikido class in Japan than a bad Wing Chun class. But don't bother with Karate cos its rubbish compared to Kung Fu !!
And have a great time. Tokyo rules !