Hmm, there is an approximation that can be done which will probably be good enough. The point 2 is misleading; what is seen in the UI is a blend of the background + layers. You can't separate them as it's already a final image, if that makes sense. But you could in theory extract a transparency value by using a black and white background version of the composite image. Then do some math magic and approximate the bgra values.
I've added the option in 1.1.0, though it's a bit awkward to use (found in File > Export as PNG...).