I think access to the Chat-room should be limited to forum members only, this can be done by either hiding the link in a members only page, requesting a login using the forum name and password or similar.
One possible problem I see though the topics and discussions in the chat-room might soon go out of hand, 2 guys wanting to chat about a plane kit, 3 about an armor kit and we get lots of cross-chatting and static.
Other forums I have been to assigned chat-times twice a week. The Chat-room is always open but at those times the chances of having people in it are increased, natually this might be awkward for members not in that particualr time-zone.