Bob,
A I read PDH's description, there are no parallel conductors as defined in 310.4. The 'white' conductor is the neutral feeding the string of lamps. The 'red' conductor is the hot leg feeding the string of lamps. The 'black' conductor is the hot conductor that feeds the 'red' conductor, but from the _far_ end of the string. Net result is that the path for current through all of the lamps is equally 2400 feet round trip, so all lamps would be equally dimmed by voltage drop.
This trick could even be extended to multi-wire circuits by using 4 conductors. One hot leg and the neutral fed at one end of the string, the other hot leg fed from the other end of the string.
Another possible use of this trick is that you end up being able to measure the voltage drop at the 'head end' of the circuit. I suppose that one could use this measured voltage to properly adjust a variable buck/boost transformer, and provide a nice fixed 120V all along the circuit. I don't know if there are any listed systems that would do this.
It is unfortunate that something similar to the 'Lightbulb Voltage Regulator' http://www.cs.indiana.edu/~willie/lvr.html is not made for mains use for incandescent lamps. I know that there are voltage regulating transformers and the like, but an incandescent lamp does not require smooth sinusoidal power. Something as simple as a common triac dimmer, but measuring the voltage and maintaining a _fixed_ output voltage, would permit a system to tolerate quite a bit of voltage drop with constant light output.
-Jon
