I like the idea of the chat room as well but agree with you iceman that posting may become less and the chat info is lost.
How about an alternative like a notice on the home or forum page for members to put up the times they are most likely to be cruising the site and have a posting area set aside for them to "chat away". If its just a general chat with little useful astronomy info in it the posting you could clear it later so as not to clog up storage space or if it does contain useful stuff store is as per the end of the next paragraph.
Or again alternatively set a particular topic that may have wide interest and nominate a time for members to login and participate in an active bullitin board type discussion. At least that way information wont be lost and it will almost be real time (might be hard to keep track of the flow of the discussion at times. Any discussions could then be held in a separate catagory for eg "Archived Bullitin Board Discussions" or such like.
Just a thought
Paul