Solved! Go to Solution.
There is an API out there but is apparently lacks the radio features: https://developer.spotify.com/technologies/libspotify/
However, there is at least one third-party app that uses the radio function so it must somehow be usable for developers: http://itunes.apple.com/us/app/spoton-radio/id471762418?mt=8
I wonder why Sonos did not manage to get this working after such a long time... :-/