cosworthbg, DO NOT rely on a knock sensor to control ignition for nitrous.

Even when working it will NOT react quick enough to prevent damage
under heavy load.

On a cossie (weber ecu) system, the ignition can be retarded very easy under nitrous
as there is an octane adjustment plug that can retard the ignition by 2,4,6 or 8
degrees by linking two or more wires together. (when NOS is active)

However, This is VERY chip manufacturer/ECU type dependent and will have to be
live tested under load by someone who knows what they are doing to
prevent "liquid pistons".