Yeah, there's some that can be done through system actions (which I think that is) and it sounds like custom actions have been implemented by selected partners, I just mean they need to open up custom actions to enable more general app-specific integration.
I can say "Open Spotify" and it will open the app. Then I have a button on the helmet that sends the Play command. But I can't do anything robust like playing a specific artist.
Perhaps if I used Google Music the integration would be built out.
On my Nexus 6p saying "OK Google play 'artist'" will open Spotify and start playing the top songs of that artist. This does not work to play specific playlists though.
Real easy to say: "Okay Google... navigate to California Academy of Sciences."
What's missing for me is spotify/app specific integration.