DALnet, Undernet and possibly EFnet all have "/who limits" if your query will generate > X results, it is trimmed to X results. On DALnet I believe X = 250. So if you are in #chan and it has 1000 users, and you /who #chan, you will only receive a response for the first 250. That means the other 750 users, you have no way to determine if they are away or not; again, this is one of the many reasons why I said it is impossible, you'd be kicked off the server for flooding long before you managed to obtain any useful information.
