Hi Stewart, welcome to the forum. Instead of the metal having to attach to the rope is there any way you can count the number of turns of the pulley? A short shaft with a lump on it and a counter.
Obviously you could crimp a band to the rope but the "the exterior of the rope is subject to excessive wear" is going to bring in repeatability issues. OR, if you can put the crimped-band further back on the rope so it is not involved in the slurry or the cause of wear.