You're making it sound more complicated than it is. Just put a post up along the lines of 'free slaves or I'll eat you' and Outcast will do the rolls. If they agree (which I reckon they will as the Zanzibar went about freeing their slaves around this period) then you have no casus belli on them, if they don't agree, then you do your steamrolling. I don't think it's too difficult, unless there is stuff you're not telling us which makes everything so much more complex.