I've looked into this a little and from my understanding..
A burner or co2 generator completely burns all the gas efficiantly creating pure co2, the exhaust from your furnace or waterheater doesn't burn the gas very efficiently so it exhausts a mix of CO (carbonmonoxide) and co2 (carbondioxide) the CO would be harmful to you as well as your plants and this is why so many ppl recommend against it.
The CO2 is heavier then the CO (44 vs 28 ), I'm currently looking into a way to easily/cheaply separate the 2 gasses.
Another option would be to somehow "purify" the channeled air into pure CO2, I'm not exactally sure how...this is all in theory but maybe runnning it through a heated line (yeh probably dangerous) or some oither means to heat the channeled air to combust 100% of the CO.
I STRONGLY reccomend that anyone that experiments with any co/co2 exhaust properly test the air regularly to make sure you don't kill yourself, or your plants
ideas? input? flames?