I would also go with some kind of thechnological implant, something tht would look and feel de-humanising, which is partly what (real) slavery requires. The less a society can identify with its slaves, the easier it is for them to 'justify' them. There are other ways that societies have done that in the past but you need something with visual impact. Something that suggests an introduced technological dependece might be good. The masks are a step in that direction.