You cannot easily modify ML to send specific commands by USB. If it was tethered to a PC you could intercept those signals. I don't think this is the way to go.
Binooetomo has a great idea. It is much simpler and will work.
I choose to move the slider at a constant speed. It can be very slow using a stepper motor. Even if the time lapse is an hour interval you can slow it down enough so it only moves a few cm. As long as it is mechanically smooth with a string to absorb the shock of the individual steps, you can expose up to about 1 second without blurring. In this case there is no need to get feedback from the camera to Arduino. You can still use the ML intervalometer.