EDITED: Explanation of solution I found (it’s a work-around).
I posted a question on how to create custom ODT tags for built-in modules, like Expedition (Shipment)
The reference at https://wiki.dolibarr.org/index.php/Create_an_ODT_document_template has a list of pre-defined ODT tags and shows how to create custom ODT tags.
I looked through the PHP code and to the best of my analysis, the above documentation for creating custom tags only applies to tags for custom built modules, not the built-in modules.
I did discover that the built-in tags are generated in the file htdocs/core/class/commondocgenerator.class.php
I have successfully added a tag by modifying commondocgenerator.class.php
It’s not the best solution, because you have to retrofit your changes every time you upgrade Dolibarr, but it works.