The Foreword to the First Edition of Introduction to Objectivist Epistemology begins like this:
(1) This series of articles is presented by ‘popular demand.’ (2) We have had so many requests for information on Objectivist epistemology that I decided to put on record a summary of one of its cardinal elements—the Objectivist theory of concepts. (3) These articles may be regarded as a preview of my future book on Objectivism, and are offered here for the guidance of philosophy students (p. 1).
Each sentence here—in fact, each clause—tells us something significant about the content of “Objectivism.” I’ve added the reference numbers.
Comment on IOE paragraph 1, sentence 1.
Comment on IOE paragraph 1, sentence 2.
Comment on IOE paragraph 1, sentence 3 (forthcoming).
The Institute for Objectivist Studies 
I remember the first time I read the above passage in the foreword to the Introduction and sensing that there was something rather wrong with the idea of a philosophical theory about which nothing had yet been written. Was she suggesting that her theory of concepts had previously been conveyed by oral tradition? The idea is not without precedent in philosophy.
I agree with the spirit of your comment. I haven’t had the chance to polish the little commentary I’ve sketched out on this passage, but when I do, I think you’ll see an overlap in our views.
My point in it is slightly different from what you say. It’s to suggest that the passage very clearly implies an “open” as opposed to “closed” conception of Objectivism. It makes no sense to suggest that Objectivism is “closed” in Peikoff’s sense if the main statement by Ayn Rand of its epistemology is a “summary” of one cardinal element of “the” theory. What theory? Well even if you sidestep the question you’ve raised, the other “cardinal elements” would have to be elements of Objectivism. She didn’t work out any of those elements in print. Suppose I do, then. Or suppose you do, or somebody else does. Suppose the content I (you, she) generate(s) is identical to what Rand would have generated. That is not a metaphysical impossibility, after all. Is that hypothetical theory not Objectivism? If not, then what is it?
She is the one saying there are non-actualized cardinal elements of her theory. Ex hypothesi, such an element can come to be by somebody’s efforts. If she is the one saying it is Objectivism, and Peikoff’s view is that her views constitute Objectivism, then as a metaphysical fact, if such an element comes into existence, it must be Objectivism when it does.
But I think you’re right to think that there’s something wrong with the idea of a philosophical theory that hasn’t yet been put into print. What I would say is that such a theory exists in potentia. A very brilliant person may well work a lot out in a purely oral fashion. As you say, there’s a precedent for that. Some of it may have been worked out in journals or scratch paper we don’t have. Etc. Etc. I’m told by a knowledgeable person (Allan Gotthelf) that Rand thought long and hard about objections to her views, and had worked-out answers to common objections to them (e.g., to The Objectivist Ethics). I’m fully prepared to grant that. But if the worked-out views don’t exist on paper, in polished form, what he said about her is just someone’s highly fallible memory of her. Maybe it’s right, maybe it’s wrong. Maybe it’s right about ethics and wrong about concepts, etc.
Suppose it’s right about concepts. Then her theory may well have been worked out in more detail than we realize or can grasp at face value. (I do think she was suggesting that.) But it may not. Both possibilities have to be taken seriously.
Incidentally, there’s a precedent for this in Rand, as well. The opening to her Conflicts of Interest essay in The Virtue of Selfishness opens with the complaint that students of Objectivism don’t seem to be able to grasp the non-conflicts principle. Well, the statement of the principle in print (prior to that essay) takes up all of three sentences in Galt’s Speech. That is either a reference to an oral tradition or a demand that everyone grasp the principle in three sentences. Either way, it’s unreasonable.
Irfan