That feature isn't part of WordPress, but I can bolt on something. Or here's another possibility: When full access control comes along, you'll have the power to create private forums whose membership you control. You could have private conversations that way, too.

Ya know, Eri, you've had a significant influence on the design of this software. One section of a technical white paper I wrote on access control is titled "Eri's Law", and sets forth the following policy:

A change in system configuration should not make a private post public without the author's permission. That is, people post each message with a certain expectation of its privacy, and we would violate our fiduciary trust if we made it possible to disclose a private post by changing its circumstances. This means each post must record the security configuration at the time of its creation, and that should be treated as the highest authority when determining post visibility. (I call this "Eri's Law" after the author on the HabitableZone who protested when a forum visibility change would have made her work in progress and draft reviews, posted when the forum was members-only, open to the public. She has an absolutely uncontestable point about her expectations of privacy.)

I've never exaggerated when I've said that I look to Zoners as experts on discussion forum life and operations. I really value your feedback.