I've wondered about this: there are a good amount of forums and blogs run by American and European dreamcast fans that are still active. But, I've yet to see a Japanese counterpart to this, despite searching in Japanese (found dead BBS and blogs that haven't been active for years).
I speak Japanese and I'd like to be an intermediary between the US and Euro DC enthusiast community and the Japanese DC enthusiast community - surely it should exist somewhere right? If there were a Japanese counterpart to Mayu or Adam Koralik, we could exchange all kinds of information about our favorite console and cover potential knowledge gaps.