If you put the CO2 into the intake "upstream" of the intake air measuring device (MAF, MAP sensor, carb venturi) then the fuel system will see the CO2 as air. It will add fuel to go with that air--which will result in a rich mixture. It may very well be richer than the O2 sensor downstream can cope with. So you'd need to add the CO2 "downstream" of the air meter. That will be very difficult with a MAP-based injection system (such as older Hondas have), because pretty much the whole manifold from the intake valves to the throttle plate is the air meter... Easier with an air-flow based system, which I think most cars today use.
The drop in intake temperature will also cause a richer mixture, as the fuel system will "see" colder intake air, which is denser and needs more fuel. (BTW, cold air intakes generally hurt fuel efficiency unless the "regular" intake temps are very high indeed. Though there are exceptions, as always!)
The CO2 is very unlikely to be dissociated in the combustion process. It is one of the products of combustion, after all.
Another idea would be to find a nice warm source of CO2 and add that into the intake, down stream of the air metering devide. You could probably tap it from the exhaust, in fact. Perhaps you could call it "Exhaust Gas Recirculation"?
...Sounds like the OP has found a nifty complex way to re-invent EGR, only in a less controlled fashion.
-soD