Not really suitable for the purpose, in my view; by the time you plait p'cord thick enough it will be very heavy and I think the surface texture is wrong for a lead rope. They've been traditionally made from either leather, some sort of webbing or most popular of all is a loose lightweight plaited cotton..........these are strong, light and wont damage either your hands or the horse's skin in the event of a mis-hap........the other thing to think about is that p'cord is too static,i.e, lack of stretch and would damage a tied horse in the eventuality of the frightened horse jerking or pulling when tied; it's also difficult to tie p'cord quickly and securely which is prerequisite in a lead rope.
It was always drummed into me by people who knew about horses - soft rope, soft hands and soft voice = happy and sound horses................................atb mac