this is done through a bot in the channel usually which generates the online users list and/or stats.
often this is done with an eggdrop bot, mIRC script or irssi script. but ofcourse there are many other ways...
I don't know any IRCd which supports something like this natively though and I don't think many would implement such a thing but it is possible (maybe as a channelmode for instance).
so I would go for the bot method :-]
(we use an irssi 'bot' which is running in a shell background process which generates a userlist and stats and writes that to a directory of our network website)