Just an update...Here's the response I received from Innuos:
Ethernet and USB are very different outputs, it's not just a matter of changing cables. If you want to connect via Ethernet, this means the ZEN works only as a server and sends the file for the Gustard to play. This is done via a protocol called UPnP. You can turn the Zen into a UPnP server by going to Sense App > System > Settings > UPnP. Choose the Native UPnP option. However, this way you won't be able to use Sense for playback, you will need an app such as BubbleUPnP on Android or mConnect on iPhone/iPad to browse and play your music - or if the Gustard has a app then use the recommended app from Gustard.