Hey, found it!

Well, not the cause, but the effect.

The .ply's in the Resources/objects folder of all black parts have their normals inverted.

When I re-invert those normals using MeshLab, and replace those .ply's before restarting LuxRender, they appear to render fine (I've only just restarted LuxRender, so I'll have to wait a bit to see if indeed all goes well).

So it looks like either Reality or LuxRender f's-up the conversion of the original .obj's to .ply files for these particular parts. But why?????

What I have now is a workaround, not a solution or a method to prevent it from happening again. But it's a step forward.

Cheers!

Erik