Photo below shows everything included now. When the system is running off the batteries, the low LED circuit shows the battery state when the case is opened, one of the LED's is wired to the front of the case itself so when that is illuminated while playing a game, it is time to save the game and exit as the system won't last too much longer.
The other indicator however is the PSone screen gets shaky, when the voltage gets too low before the screen goes off.
I must admit, I forgot that last point as it negates the need to install such a circuit! Never mind, put it in anyway!
Just need to do a burn-in test on the system and make a YouTube video - which will be doing over the weekend.