Unity _CameraDepthTexture access on Oculus GO

Does anyone know hot to get the Unity _CameraDepthTexture  working on Go?

I'm setting up the camera correctly (GetComponent<Camera>().depthTextureMode |= DepthTextureMode.Depth ) and everything looks good on PC / in rift, but on go the depth texture appears to be empty.

[I'm using a debugging shader to render the exact contents of the bugger to a quad to test this :) )

Is there some unity setting hiding somewhere that needs to be checked to enable it?  Some other setup that needs to be done on mobile only?

Any ideas gratefully accepted :)