You can still use KeyRemap4MacBook:
<autogen>__KeyOverlaidModifier__ KeyCode::COMMAND_L, ModifierFlag::COMMAND_L | ModifierFlag::NONE, KeyCode::COMMAND_L, KeyCode::F19</autogen>
<autogen>__KeyOverlaidModifier__ KeyCode::COMMAND_R, ModifierFlag::COMMAND_R | ModifierFlag::NONE, KeyCode::COMMAND_R, KeyCode::F19</autogen>
<autogen>__KeyToKey__ KeyCode::COMMAND_R, KeyCode::F19</autogen>