Here are the considerations as I see them:
Passively, mIRC can tell when a user is any channels you are in, and if so, can see them quit, part the last common channel, or join a channel you two share. Indeed, passively mIRC already tracks a user as they change nicknames in this way.
It might be valuable for mIRC to show some passive indicators similar to the Whois On Query feature, without having to probe the server.
Beyond shared channels, mIRC can probe the server for a user's connection status with /notify (ISON, WATCH, MONITOR), but this also announces to the server which query windows you have opened or closed, so it has some privacy implications.
As of this writing, all of this can be scripted without terrible issue. Except that $dqwindow echoing logic is a bit annoying.
Do you really want a query window to scroll with lots of Online/Offline messages if it's been opened for very long?
doiní things a particle can