I'd hoped this would be fixed with the launch of Witch Queen, to no avail - the Illicit Reaper Robes lack an ornament slot, meaning we can't override their appearance with that of other robes. This presumably extends to the other Reckoning armor pieces for all classes and roles. The issue was primarily exposed because the Illicit tier of armor was reissued during the Season of the Lost, and Xûr happened to sell the reaper pieces one weekend. I picked up the robes at the time, and am now stuck with their base appearance in the loadout I've built with them.