As always, the biggest problem with trying to measure the concentration of CO2 in a planted tank is that it varies a lot from spot to spot in the tank. Near fast growing plants it will be low, compared to near the input of CO2 enriched water. Near the surface it will be less than in the middle of the tank. I once saw this demonstrated, when Plantbrain first played around with his $2000+ CO2 probe.
If this device can measure the concentration of CO2 in the air in a .03 thick sheet of air on top of the water, it should not take more than seconds to reach an equilibrium with that in the water, just below the surface. But, if that volume of air is a cube 6 inches on a side, it will take a lot longer. Or, if it is a large volume with a small interface with the water it will take even longer.
I still like the idea - and Murphy's Law, like Einstein's Relativity Theory, has yet to be found incorrect
(Henry and I aren't acquainted.)