http://www.dreamcastlive.net/blogs/post/ChuChu-Rocket-Is-Back-Online/) Cool, I'm glad to see it online again, it was the first online game I played on Dreamcast! (sure it was the first online game in US).
No idea what version is supported right now, I'd say US because the PAL one probably uses dreamarena auth.
Surely Bomberman online would have been a better choice? Then again, that was only released in the states.
The Saturn doesn't have those online problems with Bomberman, because it used direct dial on the phone lines bros!!
The real game let you play against people from any region, I recall joining the Japanese servers once and getting someone from Japan to send me one of the DLC character replacements that wasn't released in the US. If it doesn't work now, they probably just need to add the proper server URL for the Japanese version to their DNS. It's unlikely that the online code would have changed much between regions.