It might go well, but you'll never get a building permit.

You want to dump the combustion fumes from a fuel burning devise into a room. Sound like you been breathing some of those poisiness gasses. Thats a sure way to kill yourself or some unsuspecting person who happens to wonder in to your gas chamber.

I'd like to see the flue gas composition of a properly-operating condensing, natural gas boiler. Acutely toxic materials should not be the real problem. CO should be very low or nil. It would be oxygen-depleted and high in CO2 and H2O. It might be very good for plant growth. High CO2 will promote plant growth in lighted conditions if not too high.

CO2 will certainly be high enough to be dangerous in a confined space. I'd summarize the effects of high CO2 this way, it first makes you stupid, then it makes you fall down and then it kills you. The hard part is the stupid part. People do not react to other symptoms in rational ways so it is very easy to fall down and die.

Yes, there are plenty of places where people might be exposed to CO2 as part of their work. That includes greenhouses and breweries. These areas are heavily monitored or the workers carry monitors.

I don't recommend your greenhouse plan without a lot of additional study and risk assessment.