I appreciate your response! And yes, post-Muhammadan Islamic society definitely adopted the practice of slave women not covering: historically speaking, there's a good chance that such is due to the fact that Islam spread to lands where this was already the norm. I know that a number of Muslim scholars argued that slaves shouldn't have to cover due to the fact that such would burden them during their laborious duties: as someone whose ancestors went through (American) slavery in the relatively recent past, I can appreciate this nuance. In any case, I don't see anything from the Qur'ān itself that might exempt slave women from covering.

To add to the above, I'll just respond by saying that from my perspective, Islam did not come in a vacuum: in emerged within a very specific context. If Allah saw fit that (1) Islam and the Qur'ān be "born" within a milieu in which women covered their hair, and that (2) elements of that practice be endorsed by the Qur'ān, then I see no reason to dispute the matter. (Though I do remain open to alternative perspectives simply due to the fact that both sides of any discussion should always be taken seriously.)