How did I just now stumble on this?
https://keenhome.io/pages/how-it-works
These smart vents are battery operated and work with Ecobee and Nest.
Pro: My mother-in-law can stop confounding Ecobee by opening the windows in her room because she likes humid 80 degree weather, SOME OF THE TIME.
Con:
1) It requires a web-based API and a hub
2) I use Ecobee, and the plugin still is rough around the edges
Honestly though, $600 for 5 rooms covers the AC zone of my house with all of the bedrooms, and would pay for itself in one or two summers.