I'm trying to test if some extracted sentences validate as productions of a particular context-free grammar expressed in a BNF dialect.

Is there an available implementation of a parser in XQuery that can do this?

In the context of Invisible XML https://invisiblexml.org/ I have heard that term "Earley" parser a lot but I am not familiar enough with the various implementations to tell whether all are "Earley" parsers, https://github.com/cmsmcq/Aparecium is an XQuery implementation of Invisible XML.

I think the various NineML tools by Norman Walsh use an Earley parser and are written in Java, there is an XPath/XQuery/XSLT extension for Saxon, so it should be possible to integrate that in BaseX, perhaps, too.