Can you set the configuration to allow on unregistered users to only view but not participate in a chat ?
Well, there is an option with the plus version: Set it up to require registration and activate the lurking feature. The guests can then open the lurking page and monitor the activity in the chat. But they won't be seen in the chat. I'll think for an add-on for your idea. Good point, I guess...
PS: http://ciprianmp.com/plus - here you n check yourself the lurking page...