With shadow buffers this will not work, since you are not in fact rendering a shadow mask, but instead a depth buffer with distance-to-occluder, which cannot be

With shadow buffers this will not work, since you are not in fact rendering a shadow mask, but instead a depth buffer with distance-to-occluder, which cannot be blended as it'd be entirely incorrect. Even if you could, the penumbra size is a function of not only the light size but also the distance from the shadowed surface to the occluding edge, which is fundamentally something you cannot know during shadow occluder rendering time.