It's a very old post but I just read it and I must ask something, how can a bottle of yeast producing CO2 can kill you? You must be in a very small room, with lots of bottles of yeast, and a very deep sleeper to actually die from it. Sometimes people overreact very easily...
I'm a molecular biology student and yes, if it spills on your plant, it could populate by eating the sugary content of your plant, but if you have a decent setup you can prevent the risk of spilling anything on your plants. Keeping the bottles of yeast away from the plants and sticking a hose or pipette in them should do the trick.