I assume you're not longer working on it, but why not just wire it so that:
- The LED is in parallel, but with the sensor voltage supply, not the chip
- Camera sensor idle voltage = low voltage for the LED (be it with stepping if needed)
- Camera sensor active voltage = high voltage for the LED (again, stepping if needed)
- little capacitor that holds enough charge to run the LED for ~3 seconds after camera goes back to idle voltage.
Good luck hacking that :)
That's basically how this works, but manufacturing electronics at a massive scale requires some more flexibility. For example, capacitors have a pretty large tolerance (sometimes +/- 20%) and LEDs have quite a bit of variety in what voltages they'll work at. So for some people the LEDs might last 3 seconds, for some they might last 5s. Using a capacitor also means the LEDs will fade slowly instead of just turning off sharply.
If the LEDs come from a different supplier one day, who is going to make sure they're still within the spec for staying on for 3 seconds?
(And yes, I have long since parted ways with Apple)
Edit:
And to add on: That capacitor needs time to charge so now the LED doesn't actually come on when the sensor comes on, it's slightly delayed!
You can't drive an LED that way in production electronics: you need to use an LED driver circuit of some kind to ensure the LED has constant current, and also to protect against failure modes. Also, a capacitor large enough to power a daylight-visible LED for 3 seconds is not as "little" as you're thinking; there's likely not enough space in a laptop lid for one of those. A driver circuit would be smaller and thinner.
Agreed, however, that the LED should be controlled by the camera sensor idle vs. active voltage.