Current detection at a distance
Probably possible. You can use a low frequency (maybe 1Hz) AC and synchronously demodulate it. The wires in the cable will be some distance apart so there should be some signal available outside the pipe. The thicker the pipe the lower the frequency that you should use (skin depth of steel at 1Hz is about 10mm). 

Major challenge- SNR and asymmetric signal- perhaps requiring 2 or more pickups.