The second guess is correct, I'm looking for a thing based on the away-status. (not the idle-time)
Performing a whois on every single user and filtering out the needed line looks quite complicated to me and would extremely clutter my status-window. I guess there is some more fancy/sophisticated way of evaluating that(?)
I read about the who command of a channel now, I was unaware of that H/G flag yet. Nice one, that seems like it's worth an approach. :-)
Still I would need to better "hide" the response from the GUI. (yeh, I'm not the über-scripter, I always need to read the mIRC-help and google a lot and wouldn't know where to start/what to search for right now, to do that in a clean way (I would probably end up writing things to files and parse them which would be awful for that kind of action xD), any hints are appreciated)

Also thanks to argv0 for the explanation. I know most comparisons like that s***, but this functionality is built-in in quite a few IRC-clients (even the IMO not very user-friendly chatzilla has that feature besides more advanced ones like xchat). I wonder if they really put so much effort in that to adapt for different ircd's etc.(?) or if those clients are hammering the server in an "unsocial" way.
As the friend-list of Chatzilla for example takes aaaages (several minutes) to recognize a user of the friendlist coming online and mIRC even recognizes that without sharing a channel with that person almost instantly (I'm still very pleased about that), I was hoping it keeps a list about the away-status, too, as it seems the user-list handling is quite advanced when it comes to detecting users. (yes, obviously I was wrong on that ;-))
Looks like the other clients have found a very interesting way of maintaining that status then. :-o