The spectrum of the shorter pulses would have a higher frequency peak show up, which would also a slightly reduced carrier (PWM frequency) peak. This is why I s

The spectrum of the shorter pulses would have a higher frequency peak show up, which would also a slightly reduced carrier (PWM frequency) peak. This is why I say it's kinda like FSK: the spectral energy somewhat shifts between two frequencies (well, really a bunch of harmonics). To decode it, with a simple setup, you could just look at the amplitude of the higher frequency peak relative to the carrier peak. In fact, you don't even need the carrier! You can just look for changes in the amplitude of the higher frequency peak.