EDIT: Well, I’ve yet again posted too soon. I checked the current Spanish and French extensions and found the “list writer internal rule responses,” which were what I was looking for. Unfortunately, modifying these is not good enough for the structural changes I need to make to the output. It seems like I’ll need to rewrite it, but in order to do so I’ll need to see its full functionality. As a result my question becomes: where can I see the source code of the “list writer internal rule” - a la Inform 7: The Program (but not outdated)?
The list-writer, fortunately, is in a part of Inform 7 that we can access and modify: the I6 Template Layer! There’s a file somewhere called Reserved/ListWriter.i6t (the exact location depends on your OS unfortunately) which contains all the internal list-writing functions, written in I6.
If you write a modified version of one of these functions, you can then replace it using a Include (- … -) in place of "section" in "file.i6t" directive. This will replace the built-in implementation with your own.