I am just now reading Brian Black's "The Character of Self in Ancient India" (SUNY Press, 2007) and on page 6 he summarizes Harry Falk as saying that "...in the Brahmanas, upanisad refers to the dominant power in a chain of dependency in which the upanisad is the final component in a list, or the final teaching that is the foundation for everything else." What I am proposing is that this is what the Buddha was doing in DA (and elsewhere), though I wouldn't use the word "foundation" for what the Buddha was pointing to as "the final teaching"; instead I'd call it "the most significant factor for liberation" or "the component we have the most control over" as the deepest and less-obviously-stated part.
Black goes on to say: "As such, an upanisad is not immediate or transparent but rather remains concealed and obscure." What I perceive in the texts is that the Buddha very consciously modeled the structure of his teachings on the methods used by the greatest teachers from his culture's heritage, and that this was, in part, what people expected of their teachers, and it follows that this would be a more effective method of spreading the teachings than coming up with something totally new to the culture (as speaking the way we speak now -- which is what we seem to expect from our Buddha -- would be pretty foreign to folks back then).
While the most current thinking of Vedic teachers in any age seems to have had a tradition of secrecy, usually handed down in private along family lines, with only the older bits having leaked out into general culture, the Buddha didn't honor that method, but offered his teachings to anyone who would listen -- he had no secret "teacher's hand" in that he would give his lectures on the deepest aspects of his insight to all comers (even getting his monks to memorize and carry out into the world teachings that Vedic lineages might well have kept to their inner circle). But just because he gave the same talks to anyone who wanted to hear them didn't mean he didn't use the methods of his culture to structure the teaching.
What I propose is that most of those interested in spiritual teachings in the Buddha's time would have understood that when he said: -"Here's a truth for you: there is suffering"- they would have begun asking themselves questions: "What does he mean by suffering? How is he defining it?" When he said he was going to explain dependent arising, and that it begins with ignorance they'd have asked "Which ignorance? And how is he defining that?" And so on for each of the links. They would have *known* he was talking about a field, and they would not have expected (the way we expect) that when he named a link, he was saying enough to actually understand the "what".
Then, having listed the whole set, they'd have recognized the structure (we don't recognize it because we don't have the context) and they'd listen to more talks, and do the practice, and in that way come to see for themselves "the what" of he was saying -- once they'd been directed where to look for it. I'm further proposing that the upanisadic method served his purposes very well, because it would cause people to put the effort into their practice to come to see for themselves what was being pointed out, and this was pretty obviously what the Buddha believed was the only workable method of gaining liberation: direct experience.