I wrote a big answer but the forums were temporarily down and swallowed it.
The power header is J6 in this picture. Ground pin is to the right; 4.75V is to the left:
Now, our job is to build some kind of circuit that:
- detects that the power header starts providing 4.75V (meaning main power is connected)
- waits half a second
- shorts the power header for a short interval (100 ms?)
- un-shorts the power header again
- does nothing until power goes away
Note that, while shorting the power header, the 4.75V that it provides out will of course be shorted to ground, so some kind of power reservoir that doesn’t go away with the short is needed. A diode into a capacitor seems reasonable.
I don’t know how much current is available on the header. Chances are, it’s a very weak pull-up, and that’s why it measures at 4.75V – the input impedance of my multimeter might be a couple of megaohms, which puts the pull-up at perhaps 100kOhm, which means it will only give us 50 uA or so.
To build a circuit that acts as I suggested above, on 50 uA of power, you’d need a very low-power microcontroller – I don’t think a low-power 555 timer chip exists that could do it. Something like a TI MSP430 might do it.
Or you can draw power from some header with actual, real power available (like the GPIO headers,) and then just make it wait 0.5 seconds, output the short for 0.1 seconds, then stop and just do nothing until the end of time.
If you have a programmable MCU board available, that should be super simple. You could do it on an Arduino, even :-)
(Also: Why is the power switch header and recovery header swapped compared to the power switch button and the recovery button? Who knows. Maybe just to make life less consistent and thus more interesting ;-)