I just released a new Chromecast related app, that let you stream your cloud Google Music uploaded by yourself and from the All Access subscription (including Radio) to any UPnP / DLNA device: XBMC, DLNA TVs, PCs, WDTV Live, Xbox 360, Sonos, Android (with BubbleUPnP), various hifi gear from Denon, Pioneer, Onkyo, and many many more.
The app is named "Cast To UPnP/DLNA for GMusic" and you can find all the details and a video in this
Google+ post
The app emulates a Chromecast device for each of your UPnP / DLNA device and let you choose it in the Google Play Music app for playback.