tagged and accessible PDF document with DocBook

Dear all,
I'm trying to generate "tagged" and "accessible" PDF documents, via docbook5 -> xsfl:fo -> pdf (with Apache FOP and docbook-xsl v1.79.1). I tried both FOP-parameters (accessibility/PDF-UA) and received a tagged PDF file. But I found out that there is no structural information inside the pdf; 'everything' is tagged as p(aragraph). The xsl:fo also lacks this kind of information. The Apache FOP Accessibility help recommends using the role attribute for tagging information inside the document:

<fo:block role="H1" font-weight="bold">I. A Level 1 Heading</fo:block>

Did I miss something (parameter, wrong stylesheet)?
Has anyone already generated accessible PDF documents based on DocBook?

Thanks, Holger