Is there any sort of switch or sensor I could tap to get an output signal when the converter is locked? Looking for something I can use for nitrous arming so it only starts spraying post-lockup.
Nothing simple. You can either poll the ECU, which will interfere with other data logging tools, and has a latency of ~100ms, or you can tap off the actual lockup solenoid line, but that's a PWM signal, so you need a little bit of logic (or a tuned R-C tank) to convert that to a clean on/off signal.