Problem is that I can't even access any of those sites to set it up to begin with. I found this: http://messenger.softros.com/ and got it loaded ok, but have to try it out on another computer on the network to see if it's actually going to work. I just hope it's secure.
That softros app looks like the right thing. There should be lots of LAN chat apps - ICQ etc. A lot depends upon how your firewalls are configured, I'm no expert, but they may trust internal traffic (only filtering internet traffic).