Writing to derived/deformed vectors?

Hello, does anyone know how to write to a “derived/deformed vector”, namely a vector which is the slurph of vectors from shape keys? I can read them as mesh data, but I can’t figure out how to write to them. I have the classic problem of needing to modify deformed vectors because at certain shape key values the vectors exceed desired boundaries. I would like to move the vectors to keep them in bounds.
I found the following in the blender source code module Derived Mesh.c - mesh_calc_modifiers routine:

/* Result of all leading deforming modifiers is cached for
   * places that wish to use the original mesh but with deformed
   * coordinates (vpaint, etc.)

Sure enough, the modified coordinates can be seen in the mesh data but I can’t figure out how to modify them.
BTW: When not using shape keys, I’m able to modify the mesh data directly.
Any assistance is appreciated!!

You cant write to derived mesh data at the moment, to do this youd either need to convert into a mesh and modify or have ‘python modifiers’

Bummer… I thought about ‘hiding’ the original mesh w/ shape keys on a non-displayed layer, copying the modified mesh data, and creating a new static mesh from that data onto a visible layer. Little bit of overhead, but time isn’t a huge factor. Everything is currently being done in Python.
What is a python modifier?
This simple test code works with static meshes but as you said, it does not work with shape key based meshes:

 obj          = Blender.Object.GetSelected()[0]
 mesh         = obj.getData(mesh=1)  
 for vertNum in xrange(len(mesh.verts)):
  mesh.verts[vertNum].co = Vector(0,0,0)

BTW: One can view some of my work on youtube under username screamatthewind