Currently for automatic export of different texture variants (e.g. different diffuse textures), it's required to map the texture to at least one (1) triangle/quad/poly (since BMesh is in blender, 2.62+).
An alternative would be to add another UV layer but that meant that redundant data had to be maintained in blender which is highly undesirable and difficult to keep up to date and consistent.
Therefore the only solution has been to create a preferrably non-shown mesh triangle/quad/poly which has the texture variant mapped to it.
Ambient occlusion (ao) textures already need a separated distinct UV map layer.
Bump / Parallax and Specular are not created automatically though that might be possible using Crazy or Insane Bump via a bash command. Cross-platform might be the difficulty here.
Thus only AO and diffuse textures can be automatically generated in the variants (though we might add an option to assume that a bump map and/or specular map is provided).