Like many Onkyo receivers, my NR656 has a second zone. It is like a build-in second receiver. It can turn on/off, select sources and control volume, completely separate from the main receiver. It can be controlled using the official app.
It seems like an ideal solution for TTS, given it won’t interrupt any playback on the main channel and build in support for Chromecast audio is on its way.
I found that the mentioned python library indeed mentions the zone 2 options. I also came across this github. Seems some people are working on the mentioned library, albeit with i different purpose in mind.