If they really wanted to, they could probably set up an IRC network/server on some unusual port (not 6667, for example). Then, if they block all the usual IRC ports, you would only be able to connect to the network/server that uses that unusual port number. For all intents and purposes, it would be an internal chatting system. Note that whoever was running the network/server would need to set some form of ban on all addresses that are not internal so that only internal connections can access it, or you may have random people connecting from outside your company.
Although I haven't ever used Rendevous, I do use Trillian and have seen it. If it is available outside of Trillian, it could do what was needed. You wouldn't be able to use it within Trillian because Trillian can access all the other networks, so could go outside with AIM/ICQ/MSN/etc.
