Is there documentation on the protocol for registering as an available endpoint device for a given user, and receiving playback commands?
As I understand it, the Alexa.Discovery interface of the Video Skill API handles discovering devices, and players would count as devices:
Your Lambda code would need to respond to an Alexa.Discovery request with a list of the devices, indicating which interfaces they support (Alexa.RemoteVideoPlayer, Alexa.PlaybackController, and Alexa.ChannelController). It should then respond to requests of those interface types directed to that particular device by sending a request to that device to handle the request from Alexa (search for content and start playing it, pause/resume playback, or change channel).
This is basically very similar to how Smart Home Skills work, with Alexa sending a discovery request to find the smart devices supported by your skill and then sending requests to control those devices.
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