I did something.

• I used quads instead of triangles and it work without problems.
• The test is only a cube with six faces and a plane with one face, each face has its own texture mapping.
  The problem with seams is serious, I have split the cube in six faces to preserve each face uv and then included in the geometry the normals of a welded cube. The 8 welded normals transformed in 24 normals.
  The cube renders in Lux as it was a welded cube. Some strange thing happens at some vertices, probably has something wrong with the normals (I did the normals by hand).
  The worst thing is that Lux is very slow, the scene has only 7 polygons and I lost the patience to wait the render finish.
  I think that is not worth for me to continue, unless LuxRender improve a lot the speed.

Stupidity also evolves!