You can hide responses from /who and /whois by halting the RAW events associated with those. Of course, you'll want to conditionally halt them because you don't want to hide them all of the time.
Considering how rare it is for people to set /away these days compared to the past, I'm not sure it's really worthwhile, but that's of course up to you.
As far as other IRC clients, I'm not sure how they do it, but because they are mostly all a LOT less used than mIRC, they can all hammer the servers and the servers probably aren't going to mind too much. If mIRC started doing that from every client out there, the servers would probably have a fit because there are so many mIRC clients out there. That's assuming that they do "hammer" the servers to maintain away statuses. Perhaps they only update away statuses every X minutes or something like that. Not accurate, but easier on the servers.
