I still have to meet someone that hasn't registered to Wikipedia but knows that you can watch (subscribe to) pages and receive notifications of its changes. And I believe a nice % would just register if they knew, in order to follow some page(s) they are interested about.
What about having the same UI with star icon or "Watch"link also for anonymous users? When they click (curious or confused) a dialog would pop up saying:
"Sign in to receive notifications about changes in this page. Not a {{SITENAME}} user yet? Register!"
This is no final copy. :)
Version: 1.23.0
Severity: enhancement