>>722
PowerShell 4以上 (U+10000以上の文字、結合文字、異体字セレクタに対応)

foreach ($s in "HELLO WORLD", "HKFADMPS㉔@IL㉒RCNJG㉓EBO㉑Q", "魚と花󠄁か゚𩸽") {
  $c = [Globalization.StringInfo]::GetTextElementEnumerator($s) | sort | unique
  $h = @{}
  0..($c.length - 1) |% {$h[$c[$_]] = $c[-$_ - 1]}
  -join $h[[Globalization.StringInfo]::GetTextElementEnumerator($s)]
}

[実行結果]
LOHHEW EDHR
OLQ㉓SJGD@㉔NKBE㉑IMPAR㉒HCF
と魚花󠄁𩸽か゚