I've been thinking about building a laser graphics system to write on clouds on nights when there are stratus clouds.

I'm worried about lasing an airplane (I am not too close to the airport but not that far either) and was thinking about hooking up an ADS-B receiver to tell the system to keep the beam away from airplanes, even if they are on the other side of the cloud deck.

A system like that, funny enough, is supposed to get approval from the FDA (Food & Drug Administration) because the FDA regulates lasers. If aircraft are involved the FDA asks the FAA what they think about it. I'd hope that they think that protective system would be good enough.