Really though, if you’re already spending money on a smart switch, you should probably just replace the outlet, other switch, and faceplate with decora (rectangle style) versions while you’re at it. It’s only going to run about $10 total for all those. Plus you can then use a smart switch that meets your firmware needs.
I’ve seen this one which may fit what you’re looking for. No idea about open source firmware though.
This one might work for you too.
Really though, if you’re already spending money on a smart switch, you should probably just replace the outlet, other switch, and faceplate with decora (rectangle style) versions while you’re at it. It’s only going to run about $10 total for all those. Plus you can then use a smart switch that meets your firmware needs.