If you use GTalk already, maybe migrating to XMPP MUC (which is the same protocol) served by, say, ejabberd could be the most straightforward way. It has log configuration (just as IRC does), but it it slightly easier to set up to push logs to clients on entering a room.

Just in case: XMPP and Jabber are the same protocol. Ejabberd is a popular XMPP server (written in Erlang, if you care about compiling from source; not hard to deploy). Google Chat uses XMPP and supports both GMail users chatting on external MUC servers and non-GMail users from another XMPP server joining GMail-hosted chats. XMPP servers usually host Multi-user conference service (aka MUC aka group chat), but it is separate enough that you could just configure MUC-only XMPP server with proper logging settings without allowing any user accounts on the server and connect via existing GMail accounts.