While I only just started writing HLSL last week or so, AFAICT all that you'd need is the vector of the camera relative to an object, and the vector of the light source (sun, in this case) relative to either of them, to work out whether or not to cast godrays, and in what direction, which you can do using vector maths, and then you just blur the edges of those objects (I know you can get the edges via shader somehow) in the direction of the vector, by a degree dependant on the strength of the light source, which you can get using the brightness of the screen at the point where the light source is.
All shaders seem to be is vectors and matrices, so it's just really a load of complicated maths. If Scanti's plugin can show us how to get those three position vectors, I'm pretty sure that it's doable (though it would take me a while to get through all the maths).
EDIT:
And static shadows would probably be simpler than that, cutting out the player, just extend the position vector of the light from the static edges to the ground, and darken everything from there to the base of the static, which would be the integral of the line representing the edges of the static. OK, so it'd be a
pretty complicated equation, but the maths would work. But I'm sure there'd be an easier way of finding that area than that.
EDIT 2: I'm saying 'all', but this is actually a huge amount of work. I'm just simplifying things.