Access to stereo cameras at runtime?

I would like to change the layer masks used to render the left and right eyes. I have code do this at run-time that use to work with the old gvr sdk and works in the editor. On device however, it does not work. When I try to enumerate all the Cameras in the scene, I only get a single main camera. Are the eye cameras hidden to the unity run-time and handled in the native daydream support? If so, how am I suppose to modify the layer masks for each eye?