You can do that, it was done in a bunch of games a few years ago. Time consuming though and can't really match the detailed shapes of the occluders.
( you have your tree meshes place out, and then camera facing tri strips additively blended onto the scene manually placed to match the gaps in foliage with fixed light direction.
A more flexible and less time consuming approach could be tracing through your shadow maps for each pixel on screen, computing the shadow factor for a bunch of steps in the atmosphere. Great results.
A cheaper / simpler approach is the screen space one described in gpu gems 3 I think, where you effectively just do a bunch of directional blurs towards light direction, killing them off if they hit an occluder texel.