UUID "contained in the work" would be troublesome in many mediums (music recordings, film, paintings, for instance). Perhaps "travels with the work" would be manageable though.

And it's worth considering that every tweet, every photo on Flickr, every video on YouTube, every status update on FB, and so on, constitutes a new copyrightable work. Not everyone will care about the copyrights of their tweets, but some will. How do you copyright something for which you have no control of the identifiers? I assume you don't intend for people to hit up the service for a new UUID before tweeting and embed the token in their tweet?

(Obviously this problem would go away if e.g. Twitter itself signed onto this scheme, automatically registering and UUIDing your tweets on your behalf.)