I think I’ve figured out my issue. It looks like through a call to GEngine->StereoRenderingDevice->UpdateViewport, a different CustomPresent is overwriting mine every frame, either in dll SteamVR or Unreal Oculus, which is why I can’t step into it.
So my new question is this: Does anyone know of a way to exactly test if a vsynced frame missed timing and is duplicated? Currently I estimate using a Tick on the player controller, but that is only an approximation of frame timing. I need to exactly know that frame rate is perfect or not.