So I have had these Zombie/Cultists as allies for my Necrons for almost a year, and the idea has always been that their nervous systems were controlled by Scarabs even after they died and started decomposing. Hence, Mind Shackle Zombies.
Recently I went back and visually reinforced the theme by adding some mini Scarabs to a selection of zombies, including both my unit Champions (from Corpse Cart zombies). I have been quite happy with how they look scattered throughout the units.