Unfortunately this can't be measured on a smartwatch.

You could very easily hook this up to a smartwatch. The device attached to the nasal cannula could be a simple BLE module + 2 air flow sensors. You don't even need a nasal cannula, thats just for increased accuracy for the study. The results of the study suggest that you can just observe someone's chest rise pattern to get similar results, though maybe with less accuracy.

I get that you can collect data on a smartwatch, but having a separate device is not as convenient. Perhaps a system linked to a desktop with an (IR?) camera would be more convenient for working in the office. Or even a smartphone placed on a desk/stand.

Amazfit claim to monitor sleep breathing, and seem fairly accurate based on my observations (n=2)

Ok, but I want to measure during work.

Are you comfortable with chest straps?