This is the big constraint:

> 2) Range hardcapped at ~1m due to how ultrasound works, you can't centralize detection. Their answer is to give everyone in the household a wearable receiver, which is eeeeeeeh idk, doesn't look consumer-friendly to me.

Sure yes if you could do this with an always-listening smartphone or smartwatch that would be workable, but even then it constrains it to an occupant-activity detector.

Fixing that would require in the best case prompting an app install when visitors arrive. And still it is deaf to any other changes such as a door closing in the wind.

All possible, but feels several technosocial cycles away. Interesting to think about anyway.