That wouldn't be a useful technique
Airshow planes use smoke oil for their performances. The oil is injected into the exhaust where it turns to smoke. (Don't worry, the EPA has approved this.) Injecting chemical agents into a very hot exhaust stream would likely cause a chemical breakdown. After all, the preferred method of destroying these agents is burning. Likewise, injecting biological agents into an exhaust stream that's about 1500 degrees Fahrenheit would probably kill the bugs. This simply wouldn't be a good way to disperse chemical or biological weapons.