Such strings are illegal in XML. I see nothing in the "JSONx Conversion Rules" that addresses the problem that the strings representable in XML are a smaller set than those in JSON.
[edit] Yup, confirmed. The documentation says: "The \b (backspace) and the \f (form feed) characters are not supported in XML and, subsequently, are not supported in JSONx." So not only does this JSON->XML thing seem obtuse, but it's partial.
I wrote a rant about this point a few days ago. Seems more well timed than I had hoped. https://plus.google.com/117593568781545916065/posts/ViNzo5Jj...