This was found to work by one of the users from Google+. Basically, you need to use aVia pro as an external player with Serenity. You can then start playback of the video in aVia, and then use aVia to cast to your Chromecast device. This will point the Chromecast to direct play the video from your Plex Media Server. This will only work for those videos that can be direct played by the Chromecast as no transcoding occurs using this method.
This should work regardless if you have Plex Pass or not.
I'm waiting until the offical SDK is released as I haven't been invited by Google to participate in their whitelisting program for chromecast. So this is your current work around for now. This should work for all future external video players as well that support the Chromecast SDK.
