KeyRemap4MacBookで右Shiftとか右Cmdを単発で叩いたらアンスコにするやつ

gistfile1.xml

<?xml version="1.0"?>
<root>
  <list>
    <item>
      <name>Handy Underscore</name>
      <list>
        <item>
          <name>Command_R to Command_R</name>
          <appendix>(+ When you type Command_R only, send Underscore)</appendix>
          <identifier>remap.commandR2commandR_underscore</identifier>
          <autogen>--KeyOverlaidModifier-- KeyCode::COMMAND_R, KeyCode::COMMAND_R, KeyCode::MINUS, ModifierFlag::SHIFT_L</autogen>
        </item>
      </list>
    </item>
  </list>
</root>

~/Library/Application Support/KeyRemap4MacBook/private.xml におけばよいっぽい