I wonder...might Google Talk offer an elegant solution?
Each person could include a Google Talk ID in their signature or public profile. The software is a free download (PC only) from Google. Then, if someone wished to engage in a chat session, they could access the person's profile and proceed accordingly.
This would offer the capability to those who want it, would be off-site, hence would create no additional moderation burden, and would have no cost to those involved.
Here's a
LINK