Hi Andrew, using a variable pressure pump wouldn't necessarily work - I had the same idea and contacted the (U.S.) nozzle manufacturer, who said they'd only guarantee it would mist the mixture above 50psi, and recommend 100psi (my pump is 70psi, so it's actually delivering about 70% of the nozzle rating).
At less than that the water just pi$$es out the end instead of fogging and could cause a hydraulic lock in the engine if it got sucked into the induction system, which would be bent rods bad.
Until I added a second solenoid with a return pipe back to the water tank to relieve the residual pressure in the line between the solenoid and nozzle I was getting two or three seconds of mixture piddling out of the nozzle on a closed throttle. Luckily there wasn't enough to cause a hydraulic lock, but it was pooling in the aluminium airbox, evaporating and keeping the engine idling for up to thirty seconds after the ignition was switched off. There can't have been a spark with no ignition on so it must have been dieseling on the fumes!
I could be wrong, but a knock sensor trigger sounds a bit complicated, not really the sort of thing an enthusiastic diy mechanic could just bolt onto a sixties motor. Anyone tried it? It would be a good idea on a high-po motor.
I did buy a vacuum actuated switch that I hoped would open the solenoid under acceleration but even at its lowest setting there wasn't enough vacuum in the inlet stacks to trigger it. That's when the simple and infallible micro-switch on the throttle lever got designed in.
So, current thinking is to modify the aluminium arm that the micro-switch sits on with a pivot, threaded bar and spring from a biro so that the arm rises up with the throttle lever and the point at which it triggers the switch can be adjusted. Ideally I want it to trigger after a couple of inches of throttle pedal travel, but not cut in during high gear cruising.