I'd have thought you would have a manual switch to over ride the nitrous relay as a safety precaution?
Kind of like an arming switch, but separate to the
ecu function switch.
If theres no power to the solenoid through the high current side of the relay (or you interrupt the circuit between the
ecu & the relay) the solenoids cannot possible pulse, plus you have a hard wired way to kill the nitrous if there is a control issue.
If you used a parallel 4 pole switch (2 pairs of switched contacts), the
ecu function switching & safety could be combined into the one switch.