I have to develop an DrupalChat & OG integration for client, but there is no way to do that without hacking DrupalChat module. I added few lines to this module, to achieve what I needed.
Basically, it just invokes specific hook now which can be defined by other modules to supply list of additional chatrooms. It is possible to have a proper implementation for existing "Public Chatroom" feature using this too.
If this gets added to module, I can also publish small additional module I created - "DrupalChat OG Integration". It allows to have an online chat with organic group you have been added to.