You could use this, it still creates the temp file, then auto-loads it into an mIRC @window and deletes it.
Code:
alias cmd {
  var %b = $$1-,%a = $+(cmd,$ticks)
  .comopen %a wscript.shell
  if !$comerr { .comclose %a $com(%a,run,1,bstr,cmd.exe /c pushd $mircdir && %b > %a 2>&1,uint,7,bool,true) }
  window -n @cmd.exe
  loadbuf -pr @cmd.exe %a
  window -g1 @cmd.exe
  .remove %a
}

Sample Useage:
/cmd dir/s/b
/cmd 7za a -ttar archive *.txt