We now have a new way to write reference links:

<ref target="../../back_matter/references/references.xml#aarschot">Aarschot</ref>

This makes your references actually valid in the directory/file tree. What it says is: "To find this item, go up the tree two levels (from texts_original to anthology to mariage) then down the tree to back_matter and then references, then find the file references.xml, and in that file, find the item with the xml:id "aarschot".

You will also be able to make a reference point to multiple locations, just by using a space to separate them, like this:

<ref target="../../back_matter/references/references.xml#montgibel ../../back_matter/references/references.xml#etna">Montgibel (Etna)</ref>

However, it's best not to do that quite yet, because I don't have the rendering code set up to deal with it.