Actually, I think they did do exactly that.
They clearly banned females from doing pretty much anything to improve their lives (which means Quark is obviously full of shit when he says the Ferengi have never practiced slavery).
While I wouldn't dispute defining Ferengi women as slaves, I don't think Quark is so much being full of shit as using a specific definition of slavery (and one, actually, that I think is important to distinguish from other types of slavery or 'unfreedom'). Namely, Ferengi women aren't forced into unpaid labor - they're outright barred from productive work. While their lack of freedoms and privileges means that they certainly conform to certain English definitions of "slave", they don't fit into mold of the "quintessential slave" in the sense that their economic potential is not being forcibly exploited. Since the Ferengi don't really subscribe to Federation standards of hewmon rights, and since their entire cultural and social psychology is based around the economic, this absence of the economic component probably means that Ferengi women don't qualify as slaves to someone like Quark.
From an out-of-universe standpoint, I also think that's what the viewer - perhaps the 1980's American viewer especially - was supposed to understand by Quark's claim that there have never been slaves in Ferengi society. They never actually forced people to work without pay under threat of violence (especially not of a hereditary kind, which is considered especially heinous and is an important part of the definition of slavery in modern times). The Ferengi certainly oppress their women to the point that yes, it would be correct to call them slaves from a certain standpoint, but what Quark really meant was that his people haven't practiced that specific form/institution of oppression which usually comes to mind when English-speakers hear the word slavery - someone doing hard labor under threat of violence until the day he dies, and his children being forced into the same type of oppression, and their children after that.
As fair as I know, Ishka lives a life of relative leisure (to the point that she constantly desires to break the monotony by engaging in productive work in secret). Quark and Rom neither beat her, nor do I think there's any indication that they'd have the legal authority or the social standing to use violence to control her behavior. And while any female children would share her unfree status, that would be a result of their sex, NOT their kinship to her - her male children are free (just as female children of male Ferengi would be unfree). Her condition isn't one that's inherited based on ancestry, generation after generation into perpetuity.
I guess what I'm trying to say is that I get Quark's point. The life that I imagine when I consider the conditions that human slaves live in is quite different, and honestly, worse than, the one that Ishka leads. I'd rather be Ishka than an African-American (male OR female) slave on a plantation. Touché, Quark. Their treatment of women is still inexcusable, and their laws regarding wages, worker's conditions and labor organizations mean that, rather than a limited number of people being actual slaves, you have a majority living in conditions just above slavery. So the Federation still have the moral high ground; it's just that when it comes to this one horrible thing, humans historically did it while the Ferengi didn't.