There are some special keys that can be used with the tuner, but:
1) They do not appear in the drop-down list
2) They do not appear to be documented.
I found about the <Keycode NN\> from another forum post, but didn't find it elsewhere.
I found the <Non-US\|> after I used <Keycode NN\> and it appeared as <Non-US\|> when I reloaded the config.
Perhaps this belongs in the http://twiddler.tekgear.com/doc/doku.php?id=nonenglishchars section. That mentions HID codes but doesn't tell how to enter use the tuner for keys that don't have that key on a US keyboard.