Update on the project while I remember, it's currently stalled because the 3.5m sockets I had were mono only and couldn't provide the three connections I needed. Boards were already made and I've ordered the equivalent stereo connector so it will just fit straight on. This was the Makita board mid-assembly, before I realised that the socket was only mono.
The four cables in this picture go to the back of the two LEDs. It was easier to use 4 cables than to try and common the grounds.
When I originally fitted the boards I found that the opto-isolators were drawing so much current that the LEDs no longer illuminated. This was solved by adding a 470 ohm resistor in-line, bringing down the current while still providing a low enough resistance on the switched side.
My local Poundland didn't have the 1m 3.5mm aux leads they advertise online so I had to pay twice the price to get some from Amazon (locally even more expensive) but they're still cheap and, as such, don't have any shielding around the internal cables. Great for my use.
Proof of concept has been tested with the Milwaukee charger, however I had to write some trigger and reset code for that to stop the tune playing constantly while the green light is lit.
C8 connectors were also added to the chargers without too much issue so they're no longer hard wired to the plugs and can share a fused cable. I don't plan on having them on at the same time so won't exceed the 3A rating of the fuse or the rating of the cable. I discharged the big capacitors in these before fiddling so I didn't give myself a spicy 240V shock. I don't recommend messing with mains stuff, obviously.
Milwaukee socket went in the back.
Makita socket in the side. The Y-lead I bought has plenty of slack to accommodate this.