Edit / Note: As mIRC is starting to count the idle-duration of the channels users on YOUR join, I set the initial check straight 5 minutes after your join. There's no way to evaluate "previous" idling in your absence.
A method that might be used to get some of the people, is to whois the people on the channel and check raw 317 for their network idle time.. if their network idle time is greater than 5 minutes, then their channel idle time has to be greater than that also.
To the OP: If you make use of this information, I recommend you do so carefully, as multiple whois' in a short period of time can flood you off of the network.
