If I am using the official Twitter software, or the official Twitter extensions in my browser, or using the Twitter website to generate a new tweet, it would be a handy feature for everybody involved if the extension or the app itself was able to check the domain, see if the user has a Twitter account, and add it to the tweet. Make it a optional with a checkbox.
Not with any Twitter code the author has to embed into the website pages as they do with Tweet buttons (not everyone uses them), just check the domain root against the user database and see if anyone uses it in their profile. Maybe they have contact info on the site itself where they link back to their Twitter account page. I’m sure it’s not as easy as I think, but it’s not impossible.
It’s in Twitter’s best interest and the users would like it. I want to let people know when I tweet their stuff. And I’m lazy.