I got an idea for a plugin: basically, the plugin will sync an on-going conversation and another user (not in the convo). Whatever that's chatted in this conversation will be sent to the 3rd user.  In this sense, the 3rd user is 'observing' the conversation without being invited inside the conversation itself.

Some may consider this an invasion of privacy. However, I think there is no difference between sending a chat log after a conversation has taken place, and this (except that the latter is much more convenient).

If one person in that chat had the plugin to relay the data to the other client (like a quote every time a new message is sent) then yes it could work. However this just seems stupid and you may when you are "listening" intervene by telling people what to say when they don't want you to know. Where with a chat log you have no influence over the conversation you can now stir things up. I do not like\see a need for this plugin unless you are extremely unpopular...

...i'm agreeing with  A-Nub-is on this

ummm. why not make a trojan instead ?