Thanks for mentioning Tynt :-)
That indeed gave me some interesting ideas I could code myself. They seem to believe that the majority of copying goes through copy and paste, so for the time being I'll focus on just that.
Perhaps I could detect it when users copy an entire article to the clipboard, and in that case replace the copied text with the contents of the description meta tag, with a link to the page, and a copyright message.
If they're copying just a part of an article, I could add a link underneath the text.
And if they're copying just a few words, they're probably copying keywords that they'll put into a search box somewhere. No need to do anything to that.