There might be a way to mod in loyalty for all cultures, not just roman. The patch 1.6 hinted something at that: all shadowed now have loyalty. My guess is that you could try to edit descr_sm_factions.txt to achieve it. Try adding to all factions that they are shadowed_by the faction "slave". In order to see how the "shadowed_by" line should look, look at one of the factions that already are shadowed, i.e. western and eastern roman empire, then copy paste it but edit so that the faction they're shadowed by are the slave faction. If you try it I'd be grateful if you told me whether it worked or not