As I read this I am reminded of an old script I made using the built-in mIRC variable $comchan - pasted a long time ago - http://pastebin.com/dNQjvM4g

menu nicklist {
  Common channels $1: /comchans $1
}
alias comchans {
  var %n $comchan($1,0)
  var %i 0
  var %t %n common channels for $1 $+ :
  while (%i < %n) {
    inc %i 1
    var %t %t $comchan($1,%i)
  }
  echo -a %t
}

More primitive output directly in the channel but it has served me well showing a single user at the time.
Scanning all users could be useful - sometimes.