This is pure speculation. I've not tried it.
Since your screen saver runs on it's own desktop - you'll need a way to attach to the "real" desktop and make a copy of the pixels to display in your screen saver.
https://msdn.microsoft.com/en-us/library/windows/desktop/ms687107%28v=vs.85%29.aspx[
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If the real desktop image is updated frequently - you may need to poll for pixel changes.
It might also be possible to tell Windows to switch back to the real desktop.