Even if I was willing to say the NEC doesn't apply to low voltage and that is certainly not true, there are fire codes and mechanical codes that deal with wiring in duct work.
I doubt a foot of wire in that duct is really that big a hazard but I would still want to see it plenum rated, just for the precedent. If a foot is OK, how about 3 feet? 10 feet? 100 feet?
