Yeah, we tried that where I work, except for the part of developing our own client.
People barely used the system despite the fact that me and the other guy that was an enthusiast of the system teached them all about how to use it, after installing and configuring Gaim on their machines. Some people (friends of ours) used irssi with SSL, because we configured a special encrypted port for testing, that unfortunately we weren't able to use from Gaim because it simply would refuse to accept the certificate and make the connection. Even so, it worked and it worked good.
When I came back from my vacation, no one used the IRC system anymore and all were using a poorly configured jabber server without encryption on a machine that crashes periodically because that used the same login authentication as the corporate email account provided for everyone.
The motive, they said, was the complexity of IRC. This complexity was, hm, the same of the actual system or even less, since we had not installed IRC Services on the server.
Maybe if we had developed our own IRC client and made it the most Idiot Friendly possible, we would be able all that nifty features of IRC you said. But the few people with a clue there never have time because they're too busy fixing the mistakes of that large majority of people without a clue. Pity.