I think your best bet would be doing a launch timing graph take timing out right off the transbrake then ramp it back in after nitrous is off. Or step retard with an external timer that will see 12v when you release the brake and stay on the same amount of time your spraying then when the nitrous is done, timer goes off take away the 12v which in return would shut the step retard off
Agree. Simplest / cheapest is via the Grid Launch Retard (LR). Use it essentially as a Step Retard, by taking out what you want on Launch (Release of TB), & turn it off when you want it off. Or, the options are endless, as you can Ramp it "off" over time. Alternatively, a number of n20 controllers can do this too. I have a Leash. With a single stage on for a short period of time, the MSD LR function should do it. Using a Digiset Timer is another option to turn systems on / off. Good luck.