I've come up with some ideas that use GMR
The idea is this, place a GMR device in the magnetic loop of a transformer. In one coil (low turns-large current) place the load and the other - high turns low current place a feedback current. The feedback control circuit is designed to keep the GMR resistance at a maximum. Then you can measure the current (voltage across resistor) flowing in the feedback coil and that should be directly proportional to the current flowing in the load coil.
The issue is that the feedback controller will not know if it has to increase of decrease current because of the symmetric nature of the GMR device response curve. For this you can use multiple GMR devices and a special transformer geometry that will indicate whether the feedback current needs to be increased or decreased.
Any thoughts ?