Yeah, go to Misc & Uninstall in Karabiner, click 'open private.xml', rightclick the file and open in textedit, and replace the contents with this. This will give you a new rule which you can toggle.
<?xml version="1.0"?>
<root>
  <item>
    <name>Swap backslash and backtick</name>
    <identifier>private.swap_backslash_and_backtick</identifier>
    <autogen>__KeyToKey__ KeyCode::BACKQUOTE, KeyCode::BACKSLASH</autogen>
    <autogen>__KeyToKey__ KeyCode::BACKSLASH, KeyCode::BACKQUOTE</autogen>
  </item>
</root>