Hello,
getDocxPathQuery only allows to query by the main document body. Adding other targets such as headers or footers is a work in progress that is not yet ready to be used.
The Indexer class (https://www.phpdocx.com/api-documentation/docxutilities/indexer-parse-word-documents-with-PHP) can return information from headers and footers, but it doesn't allow to query by a placeholder.
Regards.