Try something like this:
1. One single WP install.
2. Member content all in "privatedir" behind http authentication
3. Login script calls 2 lines of code in "privatedir" as well, say "setmembercookie.php", then brings back to to standard wp.
4. WP-Shortcode in each page / post, that either pulls the content from "privatedir" or prints a "join" text. Shortcode makes decision based on "membercookie" set or not (you'll need a hack for the shorcode, but it's really quick/easy).
Doing this on new paysites (slightly more complex but that's the main idea). I believe it works like a charm, easy to maintain, simple, secure (as the real stuff is behind http authentication anyways).
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