I also have a similar requirement and have thought of
scheme: if a page is in the [[Category:Private]] category, it'll only be
viewable/editable by registered users (and, of course, the
[[Special:Userlogin]] page is protected). This is more flexible than
using categories sounds good! If I have the time, I will have a closer look at
it. But I got the feeling that the 'template hack' Brion mentioned will also
work on that (if I don't want to make major changes).