I've turned JJ's modern TOC for Stow into a nested list structure in an <abstract> element in the header which consists of items that look like this:
...
<item>Bridge Ward Without (Southwark)<bibl><biblScope unit="pages" from="336" to="353"/></bibl></item>
...
The page numbers referred to are pb counts, not forme-work page numbers (which can be wrong, as we know). You can now browse through the document in sections, from URLs like this:
?sectionNum=40&showDraft=true
The display of the document by sections has now been implemented on the dev site. It was a bit tricky, but it seems to be working OK. The (modern) title of the section appears at the top and bottom of the page, between forward and backward navigation buttons (in the same way browsing by single pages works). Where there are nested subsections, the header of the container section appears followed by a colon and the subsection.
Only complete pages are shown. I toyed with the idea of trying to show only the exact beginnings and endings of sections, but there are so many potential problems with this that I abandoned it -- marginal labels, for instance, cause problems, and it's probably misleading to display a partial page, when the rest of our rendering shows full pages.
The question remains of how/where to display the complete modern TOC. It's of course very large -- too large, I think, to be displayed in the "Page Contents" drop-down. One option would be an icon or a link somewhere prominent, which brings up a popover (a large popup which obscures most of the page) showing the complete TOC, in scrollable form, with each item constituting a link which dismisses the popup and jumps you to its section. Another option would be for documents like this to change the behaviour of the Page Contents button so that it brings up this larger popover instead of the usual drop-down.