First off...... Simon, great job with your Sonos Module! I'm hoping to spend some time with it soon, once I get unburied from work projects.
Just wanted to add this tidbit I got today from Sonos Tech services. They finally implemented the URL call to the App from iOS and Android devices. And just thought it would be a useful kernel to use as an adjunct to the full featured interface you're working on (at the very least it would be good for items that the Sonos app is needed for like setting up new units and services)
Here you all go, consider it filed under "additional info"
https://sonos.custhelp.com/app/answers/detail/a_id/1906
Rich Fregosa
Fregosa Design