XPath has some other uses, but all I've used it for is making table of contents (screen shot)
In the F&R you can only use Regular Expressions
http://manual.calibre-ebook.com/regexp.html
Using eschwartz's RE you can find H2's with class="chapter" in the sample book (pic 2)
I guess I'm not understanding why you were looking to use XPath. Sorry, but maybe you could provide a little more about what you were looking to do?