Does anyone know how Xpath works with Folding Text? The Hog's Bay product is the outliner I've ever heard of that uses Xpath, and I wondered that it was good for.

FoldingText's documentation discusses uses of XPath for queries:

http://www.foldingtext.com/foldingtext-for-atom/documentation/api-reference/

The forum is still active and is a better place to contact the owner about technical use cases:

https://support.hogbaysoftware.com/c/foldingtext