Thanks baz -
I think somewhere in that long thread is the answer I'm looking for, and this is my starting point (I'm quoting from the thread) -
"while on the subject of helping out vray, vray SR1 creates vray materials of all materials in the scene even the materials on lines (multi-coloured from CAD imports) could there also be a script to remove all materials from lines? that way vray wouldnt create any unnecessary materials"
Only after reading that comment did I realize that the two offending lines only exist in the original CAD file.