Great idea! You might be interested in my new “Privacy Glasses” plugin, although the concept is a little different (when it’s toggled on, it hides everything except the text chunk that you’re hovering your mouse pointer over). It still might be worth checking out.
For future versions of the plugin I’ll consider adding the ability to have “private pages” on which Privacy Glasses turns itself on automatically, but I don’t know how hard that would actually be to accomplish. So it’s just an idea for now!