A really useful addition to the Mask RE would be the option to specify a scene's layer, so any object added to that layer would automatically be added to the specified mask. This would help to keep the scene masks dynamic and would make it less tedious to set up masks.
I'm not sure how the layer selection process would be - it could be 'Pick a layer from the Scene Explorer' or open a list of currently present layers and pick one (or multiple) layers from the list... not sure what's technically possible.
Still, I'm quite certain many users would find that option really useful.