I think that implementing in-game chat is A-OK,but much simplier and more comfortable will be installing an IRC server dedicated specifically to MA.And adding web interface for it will make it useable even for those who hear about IRC for the first time.Reason why I want IRC more than chat is it`s flexibility,simplicity and low traffic use.

In-game chat is going to be simplified version of IRC. So it will have all you mentioned above.