I know you have a class 1 lampholder but you see plenty of class 2 table or standard lamps for sale where the cable passes through metal parts. If a good insulated cord restraint is used at the base and conductors are not exposed until they are at the lampholder I see no reason to earth it. If you are worried you could use an additional layer of sleeving, how would it ever become live if the cable is securely restrained.