There is a Youtube plugin for the Squeezebox server from philippe44. Maybe this is what you are looking for. You can find it in the Squeezebox Server web interface under Settings/Plugins.
Thanks for the tip!
The plug-in works fine and have access to my private playlists.
The only thing I still miss is the ability to stream directly from the YT Music app to max2play (like Airplay).
I’m afraid that unfortunately this is currently not possible. But I think the idea is great and I will pass this on to our developer. If enough customers have a similar request, we may add this feature at some point in the future.