Just to give this thread a happy ending, I've now found a way of reliably getting all 8 ESCs to arm when I plug in
On the main power leads going to the distribution board on the X8, I've soldered a male balance connector in parallel, and in series with the positive lead there's a 10R resistor. So now, I connect the balance plug of the 6S pack to the male balance connector first which powers the system up, and then I connect the main power plugs (with no sparking!).
This works every time, provided the system has sat unpowered for a good 30 seconds or so before plugging in the balance connector ... if you power everything down and immediately reconnect the balance plugs the ESCs don't arm for some reason. But in reality it will take a while to swap batteries anyway, so hopefully that will never be an issue in the field.
Onwards and upwards!
David