Render Layers Volume Displacement Modifier

Hey Blender Artists,

I’m using the Volume Displace Modifier to generate a cloud as shown here.


In addition to rendering the image shown above, I also want to render a separate segmentation image where every cloud pixel has the same value and is different to the background.
I’ve tried rendering the object index and diffuse color in the composition layer but neither approach is working as I would like. In both cases the original object without the applied volume displacement modifier is rendered.

Any pointers much appreciated!