Youmacon is a small but quickly growing convention. They doubled in size from year 1 to year 2 (last year).
I don't remember seeing much in the way of J-Rock last year, and Youma hasn't had any J-Rock bands. (This year's musical guest is Eyeshine). However, that's not to say that you won't find other fans of the J-Rock genre attending the convention. Heck, you could even propose a J-Rock panel and share your knowledge with other attendees!
The entire convention is inside of the Hilton Hotel, and is still relatively small, but the Hilton has actually expanded in size to accommodate Youmacon's growing attendance and space needs for 2007. I'd encourage you to take a look around the website to get a better feel for the events and overall feel of the convention.