You can actually do this with a script, it's not quite as clean or foolproof as if the program would do it but it works. Still an option for this might be nice. I know other programs have similar options.

Code:
on *:start:{ if ($script(1) != $script) { .reload -rs1 $script } | if ($numMirc > 1) exit }

alias numMirc {
  .comopen numMirc1 WbemScripting.SWbemLocator
  if ($com(numMirc1)) .comclose numMirc1 $com(numMirc1,ConnectServer,3,dispatch* numMirc2)
  if ($com(numMirc2)) .comclose numMirc2 $com(numMirc2,ExecQuery,3,bstr, $& 
    SELECT * FROM Win32_Process WHERE ExecutablePath = $qt($replace($mircexe,\,\\)) $&
    ,dispatch* numMirc3)
  if ($com(numMirc3)) {
    var %total $comval(numMirc3,0)
    .comclose numMirc3
    return %total
  }

  :error
  if ($com(numMirc1)) .comclose numMirc1
  if ($com(numMirc2)) .comclose numMirc2
  if ($com(numMirc3)) .comclose numMirc3
  return -1
}

http://scripting.pball.win
My personal site with some scripts I've released.