Unlike Chromecast, Chromecast Audio (CCA) does not require DRM for music. It appears there is already a linux solution to stream music to a CC & CCA.
github.com/masmu/pulseaudio-dlna
With that available as an example, any chance of adding it for music on OpenELEC? It would be really nice as you could then do whole house audio. This feature would certainly be a differentiator for LibreELEC.