I am abashed to reveal that I learned this from "The Da Vinci Code," but something that seems to have escaped attention:
Wolfe is misdirecting us a little when he tells us that "hierodule" means "holy slave." The more specific meaning is "temple prostitute" in Greek and other pagan religions, and there's no way that's an accident. According to wikipedia, the act of sex with a hierodule was known as "hieros gamos":
Hieros Gamos (Greek ????? ?????, "holy wedding") or Hierogamy (Greek ?????????, again "holy wedding") means a coupling (sometimes marriage) of a god and a man or a woman, often having a symbolic meaning and generally conducted in the spring. It is an ancient ritual in which participants believed that they could gain profound religious experience through sexual intercourse. Participants assumed characteristics of deities, often channeling the deities in question, and by their union provided symbolic and literal fertility for themselves, the land, and their people. This was often done by the monarch and hierodule of the dominant religion.
Well, that's TBotNS in a nutshell. The fertility of Urth is restored by the sexual union of Severian ("the monarch") and Apheta, which conceives the New Sun. (In contrast to the union between Severian and Valeria, who represents the old & raddled Urth). Yes, I know Apheta is a larval hierogrammate, not a hierodule, but let's not get inconvenient facts get in the way. Hierogrammates was an afterthought tacked on in UotNS, and besides, hierogrammate sounds a lot like "hieros gamos."
Actually, in "Da Vinci Code" Brown talks about "hieros gamos" as a kabbalistic union of male and female practiced in ancient Jewish religion, which would mate (ha, ha) quite well with the kabbalistic context of UotNS. But from what I gather Brown made that aspect up out of thin air.
And please don't go read the "Da Vinci Code" on account of this. It is a Really Really Lame piece of literature and you should read the "non-fiction" books if the whole grail/bloodline/conspiracy stuff intrigues you. You have been warned.