This is more of a question than a comment :-) but would such a change be retrospective? Ie would it be implemented on the display side, so that an old entry containing @example (referring to a Twitter user because at the time the author knew the @ was non-functional) would now be displayed with a link to the Dreamwidth account? Or would it be implemented on the posting side, so old entries are unaffected but any new entries with that syntax would get the link inserted upon hitting "post"?

That aside, I like something along the @example for Dreamwidth and @example@external for external sites.