Thanks for your reply, Jeffrey. Yes, I read the white paper and it describes yet another method for mapping styles to tags (using Export Tagging via the Paragraph Style Options dialog). Like Method 1 described in my original question, that does work in InDesign CS5.5 and results in a PDF with screen-readable headings.
What's not explained in the white paper though is:
1. Whether it's possible at all to tag headings if there are no paragraph styles. I know having no styles in an InDesign document is not good practice, but in higher education I frequently encounter documents where the author has not included styles. Typically I think the authors have inherited a workflow that includes InDesign, but aren't proficient at using the tool. Since InDesign includes an option within the Structure Pane to tag each item individually, I would expect this to result in a screen-readable document even if styles aren't present, but I'm getting the sense that isn't the case, and for tags to work they're actually dependent on styles. If it's true that tagging items individually doesn't work, then what is the purpose of the "Tag item" option in the Structure Pane? Are there use cases where one would select this option, and if so, what is the effect?
2. Is it possible to produce a PDF with screen-readable headings usign InDesign CS4? Clearly CS5.5 has made some major improvements in accessibility but for some of the folks I'm working with upgrading from CS4 isn't currently an option. CS4, like CS5.5, has options to map styles to tags, and to tag items individually. Like CS5.5, tagging items individually doesn't seem to work; and unlike CS5.5, mapping styles to tags doesn't seem to work either.
1. Without having styles it is not possible to tag paragraphs and texts automatically. It is one of the reasons why anyone should use styles for any text.
2. Nope, it is not possible to tag based on text and their styling only. This is a new feature of Id CS5.5.