Right, thanks, I'm on Linux though; I've tried it on my mobile phones, but I'm not even sure if those keys should be doing anything there.
In any case, it seems like indeed it's on the receiving end! With looking at a different program I can see the key codes arriving as defined.
So they're not being translated afterwards. I've put a some debug output here for the reported keys (and my regular keyboard for comparison) - it seems to me that for some reason some keys, including those media keys, aren't reported/parsed as available? Any idea where to go from here?
Edit: Oh and in the Twiddler config it's <VolumeUp>, <VolumeDown>, <Mute> as expected, though I had entered them via <Keycode 128> before saving.