The encoding team raised the issue of needing to break various elements around page-breaks so that the fw elements don't inherit styling from their ancestor paragraph (or similar). I've written some usefully-repurposable XSLT in /db/data/utilities/break_elements_at_pbs.xsl, and debugged/tested it with TEH's help, to recursively split elements up through the hierarchy and knit them together with @next and @previous.
This was conceptually quite tricky, although the solution is relatively straightforward once written. Recording details here because one day I'll need to reorganize or restructure a document hierarchy in a similar way, and I don't want to have to reinvent this particular wheel. 120 minutes.