for LAN, if it's free version, i am pretty sure it's suckie.
another thing, i am not so sure, things like jabber or GAim.. they can let you setup manually about dNS and pORTS.. so, probably, i am not suggesting it will definitely work, logically, it should..... setup the chat server... on linux.. on ur office router... and manually linked... but i don't really see any reliable ones... maybe u have to check it in LAN Messengers , it may probably take hours, but it should work..
since half of the linux can run on 200mhz, 128mb, 2mb graphics, it is cheapest and most effective, just u need one RJ45 cable extra..
Have a look at Jabber. There are servers and clients for a heap of different platforms. You should be able to setup a private server without too many difficulties. Have a look at their website http://www.jabber.org/
you are welcome.. it's a long shot and will probably consume alot of hours.
but it's worth the try.. i mean, if u want customized environment networking, it's the best to customize..