I've seen it done for /userhost too. It was probably Unreal.
~ Edit ~
How do you figure that using /userhost instead of /who uses 1/5 the bandwidth? If you join a channel of 100 users and send a /who request, that's just 1 request with 100 replies. If you use /userhost that would be 100 requests with 100 replies. I don't think mIRC should auto fill the IAL at all, it's just not important enough to have ALL of the addresses as soon as you join the channel. The average user would not need the addresses at all, let alone get them automatically as soon as they join the channel. The more advanced user can script it.
Seriously... just try using /who <channel> on a channel with many hundreds of users. You'll be disconnected by the server.
I do it several times daily and don't get disconnected.
