I"ve got my desired boost function mentioned in my OP working - see the video below for a demo. I can say 'Alexa, turn on Boost-*roomname' for a 15-min boost, or 'Boost-*roomname-LONG' for a 1-hour boost. Turning off works similarly. The voice feedback detailing the end-time is not from Alexa but from TTS using Amazon Polly, playing throughout my house on several Squeezeboxes. I also have it working for various groups of rooms. When the boosting ends, a voice notification is also given.