Update! Engine kill switch complete.
I decided to use the power steering signal, as it's unused in the Insight - this input is for the RSX's hydraulic PS. The way I set it up is as follows:
-Shifter button grounds EPS input
-ECU nitrous control #1 activates
-Nitrous control #1 adds negative 600 fuel units to the fuel map
-ECU momentarily activates the A/C compressor to stop the engine more quickly