Hey, we said that the printing press had made it over! (The thinking there was that even if The Conspiracy is implausibly effective at suppressing stuff, it’s run by fallible mortals who fixate on the dangers of things that go “Bang” and ignore the way that an information revolution is going to tear their little world apart, with or without black powder. After all, plenty of gamers have made the same category error.)
Was Wilberforce the first person to try and reform society by legal dictat? Dunno, I’d think that the French Revolutionaries at least tied with him. Whether we need to discuss Frederick the Great or Peter the Great may be a matter of definitions, though you can say it’s all sort-of-Enlightenment. But then you have to discuss all the pre-medieval monarchs who converted to Christianity and converted their kingdoms at the same time by dictat. Or Mohammed. Or Plato, at least in theory (I believe that his attempt to put the idea into practice was a total eff-up.) Or Confucius, or the Legalists…
Anyhow, my ears are burning… Though I’m afraid that the Megalan Empire was more Jon Woodward’s part of that job than mine, and anyhow a lot of it is taken fairly directly from 3rd edition Fantasy. I think that Jon, very sensibly, took a lot of his ideas from the Byzantine Empire, which is indeed probably the best model to look at for a post-Classical Christian empire with slavery, governed by detailed laws. Unfortunately, I don’t know much about slavery in the Byzantine Empire, but there’s a Wikipedia page…
…Which says that it was mostly an urban thing, the countryside being more feudal. Which reminds me; I think you could find serfs (or something analogous) alongside slavery both there and at least on the margins of a few other medieval domains. The thing is, though, that while a serf isn’t bound to a master, they are by definition bound to the land. (Modulo variations in Russia, where the treatment of serfs clearly sucked on an industrial scale.) So you can’t just convert most slaves - especially urban slaves - into serfs, by way of an upgrade; they need a specific patch of land to be tied to, and to be plugged into the feudal hierarchy. Serfs aren’t just slightly less downtrodden slaves; they’re a different category entirely.
The other thing you have to allow for in Megalos is, of course, nonhuman slaves. There aren’t many, but there are some, and many of them are members of species which are at least assumed to be violent and untrustworthy - and even if the assumption is wrong, they’ll have been brutalised by prejudiced humans. Just one more worry for your reformist PCs.
Oh, and by the way, about sumptuary laws - my understanding is that somebody in Roman times proposed requiring slaves to wear something to mark them out, because there were too many slaves flouncing about getting above themselves - and somebody else screamed NO!, because there were too many slaves, and if the slaves themselves could see how many, and communicate with each other, they’d Rise Up In Revolt and Kill Us All In Our Beds. It’s a basic terror for slave-owning societies; the owners know at heart that slavery sucks, and so any slave revolt is always going to get bloody and unpleasant for the owner class. So nothing that enables slave revolt can ever be allowed. Uncontrolled mass emancipation could carry a lot of the same risks, so don’t expect it to be popular with the upper classes.