Why haven't helicopter CAT III procedures ever been developed? Why is there no helicopter equivalent to air transport CAT III ILS?  This PPrUNe thread discusses that some helicopter manufacturers have tested coupled approaches down to very low visibility minima (perhaps even zero-zero), but it seems that even with the current capability levels of autopilot technology and ILS ground station sophistication, there's no way for a helicopter to land in minima below RVR 1200'/DA 100' (i.e. the "half minima" rule applied to a standard ILS, or a COPTER ILS procedure).
Because many copter autopilots are capable of flying a Cat I ILS to 200AGL, and then descending on autopilot into a hover at a pre-determined altitude above the runway. The pilot can then descend down to the runway surface without any forward visibility. The radar altimeter is used to judge distance to the ground. Because of the unique capabilities of helicopters and their autopilots, Cat II and Cat III are superfluous.