It could be using nonlinear optical shenanigans for CO2 measurement. I met someone at NASA using optical mixing and essentially using a beat frequency to measure atmospheric CO2 with all solid state COTS components (based on absorption of solar radiation). Technique was called optical heterodyne detection.
I’ve also seen some mid IR leds being sold, although none near the 10um CO2 wavelength.
COTS CO2 monitors exist for ~$100 and could probably be modified to messure breathing gases. They’ll likely be extremely slow.
The cheapest way to measure CO2 concentration, although likely most inaccurate and slow, would be with the carbonic acid equilibrium reaction in water and a pH meter.
Ultimately the reason it’s not popular is probably because it doesn’t seem that useful. Breathing is automatic and regulated by blood CO2 concentration; I find it hard to believe that the majority of the population, with otherwise normal respiratory function, would be so off the mark. Is there strong evidence to suggest this is the case?
I believe the intention in Georgism is to levy a tax that eliminates appreciation in the value of the land. This is effectively the same as renting the land from the government. You are correct in that it would prevent people from investing in land—investment in land is purely rent-seeking behavior, and benefits no one; building improvements on land (eg mines, factories, apartment complexes) that generate value, however, does.