There are some code snippets over at mircscripts.org that check the channels that you are on for IP/host matches (and which accept wildcards). I honestly can't remember if these support multiserver or not. Find them
here and
here. These are basically just handy ways of filtering the internal address list.
If the person you want isn't on a channel with you, then you're stuck with hoping they aren't invisible and /who.
PM