How about this...

The "buttons" are double-pole switches.  the first pole simply connects a
moderate value resistor across the two wires which would normally be an
open circuit.)  This gives you something not much more complicated than
the current situation, nearly failsafe, and no worse than the current
scheme at indicating trouble.  Use the second pole of your switch to
trigger a micro (PIC) to send a signal superimposed upon the wires' DC
level (as exists due to the resistor across the line.)  This gives you
more specific location information, at a higher potential rate of failure.

BillW

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