A few fixes done, and remaining things:

• The XML link now no longer appears in the More Info box when there's no XML source document (as in the case of an index page, for instance). However, this now leaves the box empty some of the time, so I'll need to think of a cleverer solution. I don't want to slow down the XSLT processing by testing for the presence of everything that might be in there.
• Old render_page.php and other .php paths are now redirected to the correct page wherever possible (in most cases, it is possible). This is rather neat. Here's how it works:

  [in controller.xql]
  ...
  else if (contains($exist:path, 'render_page.php')) then
      <dispatch xmlns="http://exist.sourceforge.net/NS/exist">
      	<redirect url="{request:get-parameter('id', ())}.htm">
      	</redirect>
      </dispatch>
  

• Annoying xmlns:exist attributes on XHTML nodes, which were making some pages invalid, have now been removed, by including the exist prefix in @exclude-result-prefixes in the XSLT root node.