I got it to work with:
Code:
//h:blockquote/h:font[re:test(., 'CHAPTER', 'i')]
in "Structure Detection > Detect Chapters at"
but it doesn't actually add chapter markers to the book, it just creates a correct TOC
EDIT: actually I think it's working, I was using Kindle Previewer to check it and I don't think I was scrolled all the way to the bottom where the chapter markers are displayed!