I cannot find a keycode for the actual android hardware camera button, I know it can be used but I cannot find it in the documentation or during any searches. If you cannot tell me how to use the button, then at least inform me on how to make a field for the end user to set up custom controls.
You would have to code through the androids native java code. a lot of functionality is not implemented into unity. I believe this is the input for the button you are looking for
public static final int KEYCODE_CAMERA
you can view all the inputs for android here.