[thaumasiotes]

> I wonder if it is possible to implement a watermark via patterns of Oxford comma occurrences

This would be a glaring stylistic inconsistency in every text produced with a watermark. You could just as well implement a watermark by doing automated thesaurus replacements on certain of the words and using the index of the selected entry as a code.

A watermark that deeply unnerves everyone who reads the text can carry information, but it tends to render the tool itself unfit for purpose.