I had been trying to find something like this about a year ago. And failed to locate it. It seems there is no such thing. Which is a tiny part of my hypothesis that Japanese internet is technically advanced but still in its infancy (see my comment in the "Japanese broadband outpace everyone"-thread).
You might be able to find something closely resembling what you are looking for in mixi.jp or 2channel. But don't expect too much. I talked to a Japanese programmer last year and he told me that communication in Japan is not so much over anonymous portals like webmasterworld.com but between people you actually know. Of course, if both live in Tokyo it is actually quite easy to physically meet and do some nominication.