If you're talking about the away status from /away, then it would require mirc to track down this state for every user on all channels you're in.
I'm not sure if with IRCv3 or something else it's possible to get notified of such events, but if it's not, I'd say this is not something mIRC should do, as you said a script would be better suited.
It's currently possible to color users from their idle time, which I use and might do the job for you.
