I'm going to take a swing at a Google Music channel over the long weekend. I'm not really a GM user (there are 20 songs in my library), so if anyone wants to help test, drop me a PM with your email address. I'm especially interested in users with large libraries who can help determine if this is even going to be feasible with large data sets.
Edit to add the install URL: https://owner.roku.com/add/nowheremusic