I wanted to have the body move when Alexa woke. This would be easiest if I used the lights on the Alexa. I tried taking it apart and attaching a MOSFET to the LED signal, but that was super tricky. First, soldering to the pad was very difficult. Second, it turned out the LEDs weren't driven relative to ground. They were multiplexed somehow and driven with special drivers, so the MOSFET wouldn't work.
Next I tried optoisolating it and piping an LED signal to an optoisolator. That way I could isolate my MOSFET circuit from the Echo circuit. This sorta worked, but only when Alexa was booting. It turned out that the LED brightness when booting is way brighter than the LED brightness when awake, so the LED on the optoisolator wasn't being driven enough to turn on the transistor part. Grrr!
Finally, I realized that the easiest solution was probably the best; hook up a photoresistor and tape it to the outside, then watch the visible light level. In the dark enclosure, any light at all would be detectable, so taping the photoresistor right next to the LED worked out perfectly!
I thought at first that I'd use this signal to drive the MOSFET for the body motors directly, but then I noticed that when playing music the lights go off but it keeps making sounds. This logic complicated things, so I decided to pipe the signal into an ATTiny to control the body motor.