so this request will require changes to HA core before the apps can make use of it.
Currently there are no entities provided by the mobile_app integration that allow for control. A similar request was made here to allow for the switch domain.
The real question is how do we manage the communication from server to device for this, websockets is going to be battery intensive. The alternative is to use the existing notification command structure so we remain battery friendly. Not really sure whats the best way but I am not a core developer and can only speak from the android side 
Edit: also might be a duplicate of https://community.home-assistant.io/t/phone-becomes-portable-media-player-for-tts-notifications/467160 ?