I see a small bit of an issue with any kind of light-based signaling: It's more or less visible to the cult too. If jade candles are involved the subsequent 'blackout' in the distortion would put them on even higher alert and betray the group's current hideout rather prominently. So whatever kind of fireworks is going to be launched maybe it would be a better idea to do so at an offset location? Close enough to inform the outside world but far away enough to leave enough of that protective uncertainty. Once, if, help arrives it can still be guided in with more precision then.

There might be another solution especially for the latter problem, but for this I'd need to know if the distortion also blocks acoustics?