I just bought an imitation ADA "drop checker" on ebay, eBay: Type1 Co2 Drop Checker-monitoring proper dosage of CO2 (item 250036844403 end time Oct-14-06 06:06:32 PDT)
and have started experimenting with it. It seems to work extremely well!

The secret to using one of these as a CO2 meter and not as a pH indicator is to use distilled water, with KH adjusted to 4.0 with baking soda, in the bulb. At a KH of 4.0, the pH that goes along with that KH when the CO2 is at 30 ppm is 6.6, and at pH of 6.6, the AP pH test kit or any other that uses that indicator, shows an easily recognizable green color. Also, slight errors in measuring KH, say +/-0.1, which is an achievable accuracy by using 4 times bigger water sample, do not affect the reading much. An error in judging the color that is well within practicability is +/- .1, and that error gives a CO2 ppm error such that the 30 ppm may be between 25 and 40 ppm, which is far better than any other method we use. This is great because it removes completely the effect of having odd tank water, with other sources of alkalinity or acidity than carbonates and CO2.

I set up my "drop checker" with KH=4.0 water, an excess of indicator reagent, which I have found has no effect on the accuracy, but makes judging the color easier, and set it in the tank about 3" below the water surface. It took about 2 - 3 hours for the color to stabilize - unfortunately at yellow! My fish were mostly at the surface gasping!

I will be continuing to use this, adjusting the bubble rate to keep the indicator color at green and will report back here later about how it is working. Right now, for the first time in months I can see "light at the end of the CO2 measurement tunnel"!