You could repurpose the lights2 buttons like you describe, or you can use the servo buttons. The behavior is a little bit different for the two.
The lights channels are initialized at 1100us, which is neutral for most single-direction speed controllers. Our reversible speed controllers are neutral at 1500 PWM, so you will have to bring the lights output up to that value before the esc will arm.
There are also servo_min, max, center, increase, and decrease button functions. Servo 1, 2, and 3 correspond to Aux1, 2, and 3 on the pixhawk. Those functions will initialize at min, max or center PWM depending on which button is pushed first. They don’t output any PWM signal until a button is pushed.