An idea to solve the problem even it cannot be done at the exporter level.
All the rendering process can be resumed to the intersection of a ray with a polygon. If a ray coming from a light source hits the face then it bounces or go in the traditional way.
If the ray doesn't hit the polygon it continue to travel until hits something, again the traditional way.
Now what happens when the ray hits an edge of the polygon, it can be treated as it hits the polygon or hits nothing depending on how was done the rendering engine.
The idea is when the ray hits an edge it will continue to travel as it hit nothing but with the difference that the direction of travel is changed a little by a random value that also can be modulated on how sharp is the edge.
As more rays hit the edge the result will be a smooth transition between light and darkness of the shadows, simulating the effect of dispersion, diffraction or Huyghens principle.

Stupidity also evolves!