Another method -- Why not use one airpod to emit a constant sinewave and then use the other airpod's microphone listen for the amplitude of that sine wave to detect whether or not the microphone hole it is covered by a finger? I think it could make for a much faster and more efficient morse input, since you can detect dots and dashes directly instead of requiring a double-press as a dash.

There's a good chance you could also just do it with the amplitude of ambient noise and forget the sine wave generator.