The closest mIRC feature is the "notify list". However, it will show you only the
online/offline status of
specific users (the info is sent by the server) because in IRC, there's no way to get a user's
global idle time... despite whoissing. And mass-whoissing (or a periodic whois of every user of a large notify list) is a bad idea, regarding both network load and the risk to flood yourself off the server. It therefore can't become an in-build feature. You can try to script it though.
If you only want to visualize the active users per channel (and refuse to scroll), have a look at "Address book" > "Nick Colors". It allows to colour nicknames in the nicklist according to their
channel-specific idle time.
Edit: Looks like I misconceived "typing" for "chatting somewhere else". Unveils my MSN experience. Nevermind...