If you don't want a chatbox, what about an IRC channel? Someone could set it up, and then PM members who want to use it with the channel name. Whoever sets up the chatbox can regulate it, by kicking out users and only accepting requests from people with 100+ posts. Anyone who is not meant to be in the channel can be kicked out within 3 seconds.
PS: I already have an IRC channel I made ages ago. If anyone wants to chat on it temporarily then just PM me and I'll tell you what to do
