While looking through a recent copy of the Stewart MacDonald tool catalogue I noticed an item that would be useful for rigging, among other things. The item is abrasive round cord. For guitar makers this is useful for holes and slots for guitar strings. For ship modellers it would be useful for smoothing the inside of holes for rigging, as well as pulleys and so on.
Mitchell Abrasives also makes flat abrasive tape, which should also prove useful for ship modelling.