Karabiner modifier for Logitech K811 Keyboard: [EJECT] will do next/previous tracks.

private.xml

<?xml version="1.0"?>
<root>

  <deviceproductdef>
    <productname>LOGITECH_K811</productname>
    <productid>0xb317</productid>
  </deviceproductdef>

  <item>
    <name>Logitech K811 Eject to Next / Previous Tracks</name>
    <appendix>Change Keyboard Eject key to Next (press once) or Previous (press twice) Track functions</appendix>
    <identifier>private.julienma.k811_eject_to_nextOrPreviousTrack</identifier>

    <device_only>DeviceVendor::LOGITECH, DeviceProduct::LOGITECH_K811</device_only>

    <autogen>
      --KeyToConsumer--
      ConsumerKeyCode::EJECT,
      KeyCode::VK_NONE,

      Option::KEYTOKEY_DELAYED_ACTION,
      ConsumerKeyCode::MUSIC_NEXT,

      Option::KEYTOKEY_DELAYED_ACTION_CANCELED_BY, ConsumerKeyCode::EJECT,
      ConsumerKeyCode::MUSIC_PREV,
      KeyCode::VK_KEYTOKEY_DELAYED_ACTION_DROP_EVENT,
    </autogen>
  </item>

</root>

Install Karabiner:

brew cask install karabiner

Add the content of this gist to Karabiner's private.xlm.

This will enable next / previous tracks by clicking on EJECT on the K811:

• press EJECT once: next track
• press EJECT twice: previous track

Update: Karabiner doesn't work anymore with macOS Sierra 10.12.
The "replacement" app Karabiner-Element seems to be working, but the Eject key is not mappable.