I realize that some networks allow you to specify a full nick!user@host in the notify list directly, but this seems like something that could/should be handled very easily client-side. Just run the /ISON as normal, and when you get back the nicks that are actually on, run /WHO on each one and get the hostname, then do a wildcard match on it. I think this feature would make the "multi-network notify" people happy as well, since their main problem is that bots are setting off their notify lists on the second server that are using their friend's nick (on the other network), but if you were checking the full n!u@h then that wouldn't happen either. Ideally, this would be implemented as a notify list of nicks, with each nick having 1 or more [email]user@host[/email] patterns associated with it, but even just a flat list of nick!user@host patterns would be a huge improvement.
It would be a VERY good feature! (at least for me)