Problem pretty much solved!
If you google it, it seems that this is an incredibly widespread problem. I found dozens of forum thread related to normal mapping seams where there is a break in the UV mapping, but zero solutions. Most threads did not manage to figure out the real problem: in the 3D modeling program, vertices in a smooth group have continuous tangents. When exporting, most formats won't store the tangents and the continuous vertices where the UV mapping discontinuity happens will be exported as two vertices, each with a separate UV coordinate. It doesn't matter that the two vertices have the same normals (but it sure does helps that most exporters will handle the normals correctly) because once you compute the tangents, the discontinuity in the UV mapping causes discontinuity in the tangent space.
I tried the fix of breaking a polygon strip in two and setting the seam in the middle, but I'm not 3D modeler and this becomes very hard with complex UV unwraps. But if you can pull it off it works.
So I decided to fix this procedurally after the mesh load. First I wrote a debug mode to detect the seam:
[attachment=17459:03_14.png]
Then I fixed the tangents on the detected seam. The fix is not mathematically sound, but it is a fair approximation for now and it greatly reduced the normal mapping seams:
[attachment=17460:03_15.png]
There is still a subtle seam, but this one is considerably less disturbing. I suspected that this is not due to the new fixed tangents, which look pretty smooth:
[attachment=17461:03_16.png]
So it must be the actual texture of the object that has a seam. First I investigate just the normal mapping effect and looks fine and almost seamless:
[attachment=17462:03_17.png]
The small seam might be because of the fix or because the normal map is not perfectly seamless. And finally I investigate just the texture, without normal mapping and notice the seam that is near identical to the one I was searching for:
[attachment=17463:03_18.png]
Fantastic! So theoretically if the edges of the UV map will get blended and mirrored in an image editor, the seam should be fixed.
I still need to try out my fix for more complex meshes.
So my question to you is as following: how did you fix this? Being such a common problem, found from Unity to UDK imports, I'm probably not the first who has encountered it. What fix did you use? Or do you use some 3D exporters that don't have these problems? If so, could you point them out to me? Blender is really not that consistent with its export tools.