Given that it sounds a deal of trouble to run the cable, I’d wonder hard why I need the pulse sensor. The emonPi should be accurate to within a few percent – from memory you have about an even chance that it’s within ½% and about 80% chance that it’s within 1%, even though the worst case error is appallingly huge.
I’ve never tested that sensor with an extension, but I don’t see why not, if it’s only 5 m or so.
But I’d keep it away from the main cables if that’s possible. We don’t know what’s inside the sensor so we don’t know how sensitive it might be to interference if you run the cable all the way right next to the main tails from the meter.