One wire signal transfer
This logic might work, might not too. Give a try.  

Try to read the capacitance of the wire with respect to the MCU ground. The wire will be floating, yes. To read the capacitance you can use the MCU ADC (microchip has a very good application notes on this).   

Try using a dedicated proximity detection ICs from ATMEL or cypress. With proper calibration it might be possible to make it happen.