If you really want a dedicated event for it, just make a NICK event to generate a bunch of custom signals.

Code:
on *:NICK {
  var %n = 1
  while ($comchan($nick,%n)) {
    signal NICKCHAN $v1 $nick $newnick
    inc %n
  }
}

on *:SIGNAL:NICKCHAN {
  ;$1 = $chan
  ;$2 = $nick
  ;$3 = $newnick
  ;do stuff here
}


Of course, for the case mentioned by the OP, this wouldn't help much because it'd still require checking the parameters within the event handler. On that note, it'd be nice if the SIGNAL event allowed the format "on *:SIGNAL:signame:matchtext" to restrict what it matches; though that would have the same problem as extending the NICK event as described above, I personally think it would be a lot more useful.

* Quietust, QMT Productions
P.S. If you don't get this note, let me know and I'll write you another