Is this a dumb idea? When would this be done? If they didn't think of semaphores or smoke signals, they could use fireworks. There are many variables they could use, and they could conserve materials by compressing a lot of information.
Information in fireworks can be conveyed by: size of display, duration, altitude, color, sounds, and maybe other features.
How does the amount of gunpowder used in a fireworks display compare with the amount in an average battle in the Napoleonic Wars? Or the amount used in all fireworks in the US in the past 200 years, compared with the amount used in US wars in 200 years?