Hmm.
The case of a shadow occluded by geometry in front of it looks like it could be solved by adapting the blur width based on examining a small neighborhood in the depth buffer of the main camera.
The shader examining the depth buffer could then write a blur radius texture, which would be used as an auxiliary input to a variable-width blur shader.
Efficient variable-width blur is the trickiest part. The first thing that comes to mind is the tridiagonal heat equation solver in the depth-of-field filter by Kass et al. (http://graphics.pixar.com/library/DepthOfField/paper.pdf, linked earlier in the thread on CommonFilters). But it is a bit complicated to implement. I might get around to it eventually, though 
Of course this is just an idea, obviously I haven’t tested it
Nice soft edges. I think it looks pretty good.
A familiar feeling 
Yes, good point