Here is how I believe private messages work… but I am not 100% sure because I did not write the code and it did not come with thorough documentation.
A) If you scroll down to the bottom of a topic page and reply using the default “Reply To:” box, your private message will be viewable by you, the admin (me), and the original topic author.
B) If you click on light grey “Reply” link at the top of a specific post, your private message will be viewable by you, admin, and the the author of the post where you clicked “Reply”.
Try it out, see if it works, and let me know 🙂
joe