You can't directly measure V because for a superconducting system, R is exactly 0, and I is nonzero. You can't easily have components in line with the current e

You can't directly measure V because for a superconducting system, R is exactly 0, and I is nonzero. You can't easily have components in line with the current either for the same reason unless they're superconducting -- and practically making switches, diodes etc for superconductivity applications is a gigantic pain. Quench protection systems exist to make the whole of the magnet dump its current uniformly, not in one tiny area, and are very complex. This is a neat extension of the idea with B field monitoring as a proxy for I.