It seems to me that perhaps it's more appropriate to do as a micro-format, and while common, perhaps not common enough to include in the base HTML spec.

The new tags in HTML 5 are semantic in terms of the structure of a document, (e.g. header, footer, article, nav) rather than the semantics of the document content. One could argue, I guess, that the disclosure is a valid part of the document structure rather than content (though it doesn't really seem so to me). In any case, it may be a bit too narrow in scope - the reason the new tags were put into HTML5 was that Opera analysed 3.5m web pages, to find what designers were using as class/id tags in actual use. If disclosure was such a common concern, it would have likely showed up in their sample.