Question: In my alternate history (1800s), I have troop hiking through tropical sub-Saharan Africa. For plot reasons, I have man-killing mosquitos, but I also have a plant that exudes CO2 and other attractants. These flowering plants are placed in the room at night to attract the mosquitos away from people. 

Evolution-wise, however, 'consuming' CO2 and giving off O2 is so deeply ingrained in a plant's DNA, it is hardly believable to be the other way around.

I'd easily hand-wave it away as some special scent, but the CO2 is critical to the plot. **Can a plant evolve to give off CO2?** Or rather, how difficult would that be to explain.

Answer: 
All living things respire, and when they respire they break down 'fuel' to produce energy, which gives off CO2. A plant makes its source of fuel through collecting water, CO2 and light, the by-products of which are oxygen (output/waste) and glucose (the fuel). At night the plants can't collect light and instead they increase respiration, creating the energy needed for the night and the rest of the next day (stored in neat little packages called ATP).

Because of this, it seems you have no problem. Since you use the plants at night, they will naturally be giving off CO2, you only need to make a plant that respires heavily at night. 

Of course, unless you want to hand-wave, you'll need a reason for this. Perhaps the simplest solution would be to have a fast-growing plant, maybe herbaceous. The rapid growth demands more energy, which means more CO2 intake during the day and therefore more CO2 output during the night.
You might also look into *photorespiration*, an error in photosynthesis which causes a plant to produce CO2 instead of O2. This only occurs when CO2 supply to the plants is low, which might happen when the stomata are closed to prevent water loss. Research suggests that photorespiration might have to do with nitrogen assimilation as well, meaning a plant in a nitrogen-poor environment might be more likely to photorespire.