I was trying this same thing and thought it was possible, likely from a mis-remembered demo video.
You CAN select one face of a poly-surface and apply another / unique material. This is weird and ironic, because you use the SHIFT-CTRL sub-object selection from Sub-D … but you’re on a NURBS polysurface.
OK, here is the happy part. You can do this BUT ONLY IF you convert the Sub-D to a NURBS. Use the command ‘ToNurbs’ and it will work. Just make sure you are done editing the Sub-D or make a copy of it before the conversion.