Updated v1.0.2, which you can find in the beta channel of plugin store.
To install from beta channel:
- Uninstall current version.
- Enable Plugins Test Mode in
http://<volumio address>/dev - Find the YouTube Cast Receiver plugin in the plugin store. Click the Details button under it and choose latest beta version to install.
- Restart Volumio.
New features
1. YouTube Music support
Yes, I was wrong before in saying this could not be done 
2. Multiple connections
You can control playback from multiple connected devices. To clarify, multiple connections must be of the same client type, i.e. you cannot have both YT and YT Music connected at the same time.
3. Manual pairing, aka Link with TV Code (YouTube mobile app only)
You can now connect with YouTube mobile app even when device is not on the same network; might also serve as a workaround for those who have problems with Volumio not showing up in the Cast menu.
Go to plugin settings to obtain pairing code, then open Cast menu on YouTube mobile app and choose Link with TV Code.
Never share / expose the pairing code!
4. Playback of private videos and music
To clarify, ‘private’ means self-owned and not made public, such as Uploads in your YT Music library. Not sure if it will work with private media owned by others but shared with you.
This version is a major update, with extensive code changes especially in the underlying yt-cast-receiver library. I would like to have tested it longer to see if there will be major issues after, say, a month, but maybe releasing it now as beta will allow issues to be spotted sooner. So, happy testing!
Only enable Verbose Logging in the plugin settings when you have an issue that needs debugging. After enabling this option, logs should be sent to me via PM. Do not post them anywhere else as they might contain information related to your Cast sessions and environment (no passwords / usernames involved, but still not a good idea to expose them).