I think you'd need close to 2000 ppm or so in the head space to get 25-30ppm in solution. I did the calculation back in 1998 for a class for CO2 rise and the impacts in freshwater. This assumes that the 2 systems are in equilibrium, but that is rarely the case.
The issue is that the sensor will be slow, just like those Drop checkers.
A pH meter and relative change in degassed vs enriched would work much better and be more responsive.
In other words, a 90-100 pH meter would likely fit the bill much better.
Worth a shot though.
Since lag time is a factor, a pH/CO2 standard could be done using this at different times through the day for checking the pH meter vs the pH/KH/CO2 chart. But the lag time and stability of the system need accounted for.