Bus Contention - Output Pin driving another Output Pin
I would personally use tri-state outputs in this situation.

Instead of driving a 0 or a 1, you could have pull-down (or pull up depending on your preference) resistors on the outputs, and drive the pin you're interested in high or low - the opposite of your chosen pull-up/-down resistor.  The other outputs should be set to high impedance so they don't interfere with the expected results.  

The pull-up/-down resistors would probably be best situated at the receiving end of the wiring harness so that it 'defaults' the inputs to a known state and stops them floating, rather than defaulting the signal through the harness.

Makes it a little more complex than a simple shift register...

Also, maybe Open Collector (or Open Drain for you CMOS junkies) would do the trick in a similar way...