Note, there are a few different ions there, you need to add various chemicals to rule out interferences by other ions, you cannot simply use some of them continuously in the tank due to the potporuii of ions we normally have in there.
Vaughn, since the Bromo blue is colormetric, you should be able to treat it like a meter using a Spectrophotometer.
There are fairly cheap ones for sale on line for 99$ etc.
You take the sample out and drop in the cuvette and measure intensity over a specific wavelength of interest. If you are interested, I have a whole listing of chemical reagents and methods to measure darn near everything, you need a small amount glass ware and scales etc, but you can do the DIY approach like many scientist and measure 200 or more different water parameters fairly well with it.
You have the pH indicator, you just need to make a pair or 3 standard CO2 references, one is the air/water at equilibrium. You knwo the KH and the amount of pH indicator, all you need is to make a calibration curve for CO2 over a 1-50ppm range. A 50ppm CO2 reference solution would be nice.