Direction guessing sounds do-able. If a voxel is visible from both front, side and bottom, it's a front, bottom, right or front, bottom, left (based on which side you take), and you choose a value.
I'll write something like this for the voxel editor.

> my solution is straight-forward. It is the same technique
> as in the old editor, I just had the wrong values. And
> smoothing simply didn't work.

> we need to know the normals that equate to the 18 sides in
> Godwin's diagram:

> http://ar2.eclipsemods.com/godwin/faces.gif

> It doesn't matter if this isn't how normals are really
> done, it is dictated by how you guess the facings of a
> voxel.

> We need:

> bottom -
> top -
> left -
> right -
> front -
> back -
> back, top, left -
> back, top, right -
> back, bottom, left -
> back, bottom, right -
> front, top, left -
> front, top, right -
> front, bottom, left -
> front, bottom, right -
> left, top -
> left, bottom -
> right, top -
> right, bottom -

> Now I know this isn't perfect, but it based upon the
> technique for guessing facings of voxel models, not on the
> actual method used by WW. A more accurate model with
> better granularity can be developed for those importing
> from a polygon model.