I created an ovrLayerType_QuadHeadLocked layer and initialized it. I'm just rendering it solid red for the moment. Such a layer has a 3D position. From looking at the Oculus samples, it appears that rotation is ignored on such a layer and the translation should have a negative Z value.
Right now the state is not good. Not only does the GUI layer not appear on screen correctly, about 30% of the time the 3D view flips upside down. It is extremely unpleasant when this happens if I'm wearing the HMD. I suspect that this may be an unintended consequence of my automatic effect disabling code based on frame rate.
The right eye also starts flashing cyan (the G3D default background color) every now and then. I think that both visual errors are separate from the GUI work that I've been doing.