Turns out it's easy to publish to MQTT in a way that Home Assistant can receive, so recent changes in the GitHub repo have added this capability.

I'll eventually need to add over-the-wire security, but I haven't yet looked for (let alone found) a key management solution that I like. Particle's is pretty cool, but is (as is natural) tied to their service. Azure IoT Hub stymied me the last couple of times I tried to set it up--it's a very 'hyperscaler' product--so I don't know if it would work.

Also, this way I don't need Internet access for things to work. Heck, I don't even need DNS--just Home Assistant and Mosquitto on the Pi 2, mDNS, and my little widget.

One way to solve this would be to move off of WiFi entirely. LoRa, Zigbee, or something similar might come with a good solution and, as a bonus, would mean less traffic on 2.4GHz (which is crowded around here).