To progress further with the initial flashlight idea, controlling the flash directly appears to work fine in the following way.
#!/usr/bin/env bash
flashOff() {
printf '%s' "0" > /sys/class/leds/led:torch_0/brightness
printf '%s' "0" > /sys/class/leds/led:switch_0/brightness
}
flashOn() { # 2 appears to be the minimum brightness value possible
printf '%s' "2" > /sys/class/leds/led:torch_0/brightness
printf '%s' "1" > /sys/class/leds/led:switch_0/brightness
}
So with a loop I can have a nice flashing pattern at the minimum brightness.
Just need to find what to hook into so I can have it only running only while there is a green notifications icon.
As for using the screen, I think the green envelope icon, possibly slowly moving along the top edge of the screen (to minimise any burn in); would be a nice option there.