Dreamwidth already has the username tag to link to other sites: https://www.dreamwidth.org/support/faqbrowse?faqid=87 It supports a lot of different forms. I disagree that @username has become a general convention. It's strongly associated with Twitter accounts, and with "@-ing" a user on Tumblr, which means they get notified of mentions whether they want to or not. If I see "@" I'm going to think Twitter.

This also seems like a back door to "let's have notifications of @ mentions," which I am personally really really not for.