It already does. I tested this with both regular On *:QUIT: {, as well as On ^*:QUIT: {

Code:
On *:QUIT: {
  if (!$window(@test)) { return }
  var %i = 1, %n = $comchan($nick,0), %output
  while (%i <= %n) {
    var %chan = $comchan($nick,%i)
    var %pnick = $nick(%chan,$nick).pnick
    var %output = %output %pnick on %chan /
    inc %i
  }
  echo -ti @test * $nick Quit: $left(%output,-1)
}

; Type '/window -eiz @test' to activate.


Does this satisfy the feature you were asking for?

Well. At least I won lunch.
Good philosophy, see good in bad, I like!