I have had both of these work to force pin 17 low after shutdown. Not sure if this will work for your situation or with any other pins.
run this from command line
sudo dtoverlay -v gpio-poweroff gpiopin=17 active_low=1
The Pi 3 is protected by the SMBJ5.0A over-voltage suppressor, which has a breakdown voltage between 6.4 and 7.1V. So above 7.1V you are guaranteed to trigger the over-voltage protection.
If the protection works as designed, you'll see no smoke: the polyfuse will limit the current to a safe value. However, 7.1V is above the rated maximum voltage of 5V ...
The Pi is rated at 5.0 ± 0.25V.
The PMIC (on modern Pi) has a MAXIMUM input of 5.5V - exceed this at your peril.
The "protection" circuitry consists of a combination of polyfuse, ideal diode and surge limiting diode, which varies on model (consult schematics for detail). The Pi3A+ has no ideal diode.
Schematics for most models are available.
With a 1kΩ resistor you can't pull the pin LOW. You will get ~1.2V which is HIGH.
Just get rid of it or reduce to 100Ω.
There is no need for a resistor; it is a protection mechanism to prevent excessive current for an unlikely combination of circumstances.
An opto-isolator would never be capable of pulling enough current to cause problems.
The pijuice library GetStatus() function should give you what you need:
Gets basic PiJuice status information about power inputs, battery and events.