[...]

-   With a custom-reference.docx modified to number section headings, 

    `# References {-}` came out numbered, too; i.e., `{-}` was ignored. 

    (A proper fix would probably require pandoc to attach a docx style 

    like "Heading (unnumbered) 1" to such headings.) 

[...]

This Lua filter might help you:

Your guess is correct; you have to prepare unnumbered custom heading styles(up to 4th level) in your reference.docx, 

also need to point them in YAML frontmatter (or dedicated extra YAML file).