Yes, Disqus conversations can be private if you use a SSO system. But you need to have something that works in conjunction with Disqus. So the huge advantage of Sitelok is that it does offer a SSO system to work with Disqus (it’s an extra plugin to work with Sitelok).
Only about 10 days ago Disqus made the official announcement about changes and now offering subscription pricing. I believe Adrian at Sitelok is still investigating how this will impact their Disqus plugin. Depending on his findings I may need to move to a different method for commenting in the future: possibly something like a discussion board (Vanilla, etc.).