on *:text:*:?: {
  var %cchan = $comchan($me,$nick,0), %c = 1
  while (%c <= %cchan) {
    if ($comchan($me,$nick,%c) == #channel) { reply with msg }
    inc %c
  }
  :end
}

Change #channel to the channel you want the person to be in to reply with the msg