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.

Comparing with flashlight on and off.

Off
/sys/class/led/led:switch_0/brightness = 0
/sys/class/led/led:torch_0/brightness = 0
/sys/class/led/led:torch_3/brightness = 0

On
/sys/class/led/led:switch_0/brightness = 1
/sys/class/led/led:torch_0/brightness = 127
/sys/class/led/led:torch_3/brightness = 127

I'll need to test if changing these will trigger the light, certainly just making switch_0 = 1 doesn't work.
Not sure if this is the correct way to interface with these.

As for monitoring for new messages I'm wondering if dbus-monitor could be used...