The fire could be the source of the power. In combination with low power switch (versus using relay), there is away to do monitor (even control the fireplace). I built Zigbee heat powered devices. It has been working great.
More detail here
There is a mode where you can disable Zigbee command therefore you would not be able to operate the fireplace from your Hubitat. However, the wall switch and status of the fireplace is still fully functional. You can use it to automate any thing much faster than monitoring the temperature. The device do measure ambient temperature which tell you the temperature underneath of the fireplace.
This project is in holding pattern for now. I can't find component at reasonable cost to build them. I am just a hobbyist. At this point, I can only share what is possible to do in automating your fireplace.
