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Quote - because of the way texture coordinates are handled, we have to cut the mesh apart at texture seams and material boundaries. There's no way of telling Lux that these are no real boundaries

Didn't I read something about it in the "Exporter developer's wish list for v.0.8" thread, over in the Lux forums?

I seem to remember seeing someone suggest an additional parameter to the mesh primitive, which would be a list of pairs of vertices to be 'sewn' (or 'welded', in Poser lingo) together. Unfortunately, I can't find that post at the moment. That's not an ideal solution from the point of view of exporter developers, but it looks like one that might be relatively simple to implement in Lux. So hopefully, they'll get on it soon.

If you export the normals the solution is very simple, you only need to create extra vertices in the xported lux files, these extra vertices have the same x,y,z values, the same normal values, but different uv values and the faces at the seam boundary on one side will point to the original vertices and on the other side to the extra vertices. As you export the normals and the normals are the same for both faces for Lux it will be two welded faces even have different vertices number.
The same for different material boundaries..

Stupidity also evolves!