Best practice for camera rotation offset in Unity 5

I think what you want is not technically supportable until Unity seperates the VRCameraTransform and the InputTracker.

The crucial thing is the ordering of quaternion rotations. You need to apply your offset after the camera orientation.

But, that said... you could *cough* hack it *cough*...

Not pretty but, parent the camera with 2 levels. First parents job is to nullify any rotations and position changes the unity is applying on the VR Camera. Grandparents job is to apply the VR.InputTracking.GetLocalRotation(VRNode.CenterEye) and add your offset.

This is working for me, and I think it achieves what you described - when I look around the horizon stays level, but I am at a 45 degree down angle. (It's a bit weird though - so it depends on your game).

To nullify the VRCam:

public class NullifyTransform : MonoBehaviour {

    public Transform t;

	void LateUpdate () {
        if (t != null)
        {
            transform.localRotation = Quaternion.Inverse(t.localRotation);
            transform.localPosition = -t.localPosition;
        }
	} 
}

To apply the VR orientation (could apply position as well)

public class VRTransform : MonoBehaviour {
	void Update () {
        transform.localRotation = InputTracking.GetLocalRotation(VRNode.CenterEye) * Quaternion.Euler(45, 0, 0);
	} 
}

Unity package attached.