Considering what C4 did for Extra Vegetables partners after the purchase, I don't think that C4 will be calling the kettle black.
Calling the Sonos App, then returning to the caller, is not hard on iOS, but the calling App must be compiled to do this. (It's just a few added statements) I assume that this is true for Androids too.
Given the complexity of the Sonos player interactions with the controller and each other, and the idea that the protocol was never formally published, I assumed that we would end up here at some point and never tried to develop my own controller for my customers. Mostly, the protocol is open (UPnP is open), but there are nagging little details that could trip up an application that attempts direct control, particularly with respect to music service interactions.
I find that customers are OK as the Sonos controller is called from the remote App, then returning to the remote control App. Returning to the remote control App is a four finger swipe or a touch in the upper left. Customers get this on the first try. Sticking with the Sonos controller also has the advantage that customers have access to any new Sonos controller features as soon as the features are released and don't need to wait for me to develop a response to the update then distribute my code to each customer.