I'm rather new to arnold, still trying to help you.
I've attached an image from my maya work space showing my solution.
1. create a aiFlat shader and assign to your desired objects.
2.create a "surfaceLuminance" node.
3.Connect it's "OutValue" to R,G and B channel off the color of the flat shader.
Do this in a render layer , make a material override and disable AOVs for this layer, render just beaty pass.
Maybe there is a simpler way but I don't know.
