I believe Gautam over at Opera may have some viewport stats he can share with the list. He'll be emailing that pretty soon I think.
I feel a compromise with a labeled "Add to Watchlist" button at the bottom of the article on low-JS devices (freeing up the menubar at the top), but keeping the star at the top on higher-JS devices (and hiding the "Add to Watchlist" button at the bottom), is one way to strike a balance between discoverability and usability in light of what seems to be pretty low practical usage of the feature on the low-JS devices.
-Adam