I have no idea what to tell you, except that I can't reproduce your problem at all.
I just ran a test with a cable materials directory (so just "SplineRope" and a "$basetexture" field), and those all imported fine without needing to change or edit anything in the VMTs, so I have no idea what was going on on your machine. Any unknown types should default to the LightmappedGeneric handler anyway...
I have to say I sort of expected you to not be able to reproduce any of my problems because for my entire life every time I have laid a hand on a computer I have had unsolvable problem that make no sense such as three entirely different discs in different conditions for my favorite video game not working on one day and stopping at the exact same spot on the loading progress bar. In fact the discs weren't even of the same version of the game because they each had subtle but strange differences such as missing guardrails or slightly different clipping for a shortcut on the final level. One included a first-person racing mode and in Europe the game has an entirely different title!
I have tested the game on two vastly different computers and 20 versions of the emulator (The game is named Speed Punks/SpeedFreaks(UK) by the way. Not asking you to fix this you couldn't it is impossible, it is just very interesting to see such problems with normal electronics. EDIT: Not saying this is a bad thing though because the quirk works both ways such as updates taking a long time on my friends computer and in a mere 31 seconds my computer downloads the whole game, it is hilarious really but sometimes it is a pain to try and fix the weird stuff that happens when I try to get work done.