I was thinking that one way to do it would be for someone official to create a journal with that username, and have the profile and the one entry say something like "Looking for help with Dreamwidth? Dreamwidth Support is located at [link]!" with a short explanation that this journal is a placeholder. For me, it's the same idea as what you find at http://username.dreamwidth.org/

That way it doesn't break the "subdomains are journals" convention, but also doesn't leave people without clues when they're looking for help.