I have found a way to modify the privatePlus plugin to partially work on wp 2.5.1. By removing a little code it can be made to allow logged in users (of all levels) to read private posts and pages. But I do not know how to change the plugin to make it work with wp_list_pages on wp 2.5.1 I do not have the time required to dig deeper into the wp code.
Now I can access private pages by direct URL but they still don’t show up in the page list. My partial solution is awaiting approval as a comment.