I've found this kind of setup, regardless of bdsm contents, quite challenging. Changing history often makes me cut out important social development steps that would however be needed to explain other behaviours, having me fill in fictional events that lead to the same results in that regard. It's easy to loose oneself in those complex interdependencies and get stuck with the story line, and I have quite a number of paper zombies resting in my drawers because of that.
That's why I've become more fond of the approach to take the current status and pick out one or two real world problems to construct a halfway plausible development. As an example, if I wanted to create a world with a reasonable number of willing slaves, I'd start with the possible sources. One that comes instantly to my mind are convicted criminals. Almost every major country has problems accomodating prisoners in a way that is affordable while still being safe. So why not let them have the choice between a regular prison sentence and a shortened and maybe in other (monetary?) ways appealing slave duty. Well-off customers could then rent those slaves and cross-finance the "conservative" convicts. Add to that a little bit of decadence having developed with the juries by having them put arbitrary constraints on the slave contracts (don't forget to stir the mix sufficiently), the author has every tool in his hands to make his world plausible.