If you're talking to me, then I haven't found much at all. I did notice a new TCG DS Game announced, (possibly), that's cool, but I digress. GameFAQs Walkthroughs have no mention of Dome Cup, and the Message Board actually only had 1 Question about it... which didn't have an answer. There was one YouTube video, (http://www.youtube.com/watch?v=nIzn6TKni2w) which did actually have a player participating as well as a comment on how they got there and a reply, but it was an uncertainty, it only adds to my belief it's related to link battles. You still have to rescue all the Grand Masters though, as Rod appears in the video.
PS, Sorry I'm getting off topic a little bit.