HomePods are controlled via the Home app on iOS, which uses Apple's HomeKit (https://www.apple.com/ios/home/). Someone wrote a Node.js service called Homebridge (https://homebridge.io) that lets you write your own plugins to show up in Home, so I did some coding and now have our Raspberry Pis that have the temperature sensors attached showing up right from within iOS! I'm going to write up a blog post going over how I got it all going in detail, because there was a LOT of trial and error.
I was talking about the HomePod with co-workers and mentioned our old audio setup and the mess of cables and how having stereo sound in our office didn't really matter, then I remembered that I'd posted about our setup twice previously — originally in 2014 (https://virtualwolf.org/media/posts/597) then the current arrangement in 2016 (https://virtualwolf.org/media/posts/434) — but hadn't taken an up-to-date one. So I tidied up, and here it is!