Taking a break from the unending boredom that is soldering small bits of wire to other small bits of wire (people still do this for hours every day?!), I have now attached the first new component to the console.  To wit and viz.  : –

It might not look much to you, but it’s a busbar to me.  This is where the reference GND/-ve/0V/wossname wires will be safely gathered in, and sent back to the -ve terminal on the 12V PSU.  First the keyboards, then the pedalboard, then the tabs, then… ah, but I get ahead of myself.