Is there any reason why you would not just covert your SubD back To Poly, and go Polygons> Mirror Geometry, then Convert Poly to SubD again? That is if SubD Mirror does not work.
Did you model something in SubD that you would lose if you converted back to Poly? I'm not sure if it made a difference. Of course you can always do the standard Duplicate function, then merge vertices, but that seems unituitive.