You can mass mail easily. Just search for "url" instead of by username or other criteria.
You could also search for users, export the returned user list, and then copy/paste users into a PM "to". Be sure your PM permissions are (at least temporarily) large enough to allow more than the standard 5 concurrent recipients.
In my opinion, both are easier than rewriting PHP (or writing plugins), unless this is more than a one-off activity.
Several ways to skin this cat.