Intervalometer output signal to control stepper motor - bramping

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I will be using a basic shoot move shoot controller but wondering how I sync it with Bulb ramping.
I have been thinking that if the stop delay period pulse could be replaced/triggered by a simple signal from the camera.  This would just leave the move timer acting. What I was thinking is whilst ever the camera is outputting an LCD video signal (ie between shots, the motor can move (within its time set point and stop). As soon as the screen/mirror locks up the motor stops if it has not already. Resets and then video comes on and the motor starts.
This video out signal idea came as a result of trying an external monitor today. Hope someone smart can run with this... or enlighten me of how to do it another way (preferably cheaply