My first thought was running an IRC or Jabber server on the LAN. I haven't tried that myself but the guides are all over the Web. Do a "yum search" and you'll see there are a bunch of server software for IRC or Jabber in the Fedora repo.
After the server is up, tell all clients to connect to it with something like Pidgin, which can be configured to pop up the chat window whenever a new message is received.
from rlyeh import cthulhu