I'm not sure I know what you mean but if you're talking about effectively using a switched AM system with an audio carrier i.e. the audio switched on and off to make the PPM signal you actually want then you'll need to use a very high frequency to get any accuracy when you decode it for the Tx. E.g at 1Khz a 1ms pulse will be only 1 cycle of your audio carrier and 2ms will be 2 cycles. That means you'd have no analog control at all just on/off. Even at 20KHz you won't have very much control. And you'll get losses in accuracy in whatever you use for your demodulator too.

But perhaps you're talking about something completely different. I know that driving a port works but your scheme might too. Why not try it ?

Steve