Remapping Pins for Non-printer

I'm having some issues with figuring out how to remap a pin and could use some help.

I'm putting together a 3D scanner that is based off of this system ( [github.com] )
I've had to make a lot of adjustments, but I'm almost done and just need to get my shutter release working.
For the shutter release, I've put together something similar to what's done in the following link and got it working using the basic blink sketch on an Arduino Uno ( [baltimorenode.org] )

For the scanner, I'm using Marlin on a Mega paired with a ramps 1.6 board. In the original scanner's sample gcodes, they used pin 32 to send the signal to trigger the camera.
Here's a little sample of that code:

(-------------------------------Rotate 1)
G4 P500 (Pause before camera trigger) 
M42 P32 S0 (Camera trigger ON)
G4 P600 (Pause before camera trigger)
M42 P32 S255 (Camera trigger OFF)
(-------------------------------Rotate 2 22.5)
G0 E6 F400
G4 P500 (Pause before camera trigger) 
M42 P32 S0 (Camera trigger ON)
G4 P600 (Pause before camera trigger)
M42 P32 S255 (Camera trigger OFF)
(-------------------------------Rotate 3 45)
G0 E6 F400
G4 P500 (Pause before camera trigger) 
M42 P32 S0 (Camera trigger ON)
G4 P600 (Pause before camera trigger)
M42 P32 S255 (Camera trigger OFF)

Unfortunately, I'm using a RepRapDiscount Full Graphic Smart Controller, and so all the pins from the AUX-4 group are occupied.
I'd read examples of using open endstop pins or other AUX pins as alternatives, but pointing directly to those pins doesn't work.
I've seen that remapping pins in the pins_arduino.h file may be where to sort this out, but I don't really see a clear explanation of how to do it.
Any help or guidance would be greatly appreciated!