customize xref to title

I would like to allow xrefs to titles. I have customized my dsl stylesheets to generate the page number on which the title appears, so, for example:

<xref linkend="A_SECTION"/><xref linkend="TITLE_OF_SECTION"/>

Produces the text:

Section 6.7 (p. 68)

The stylesheet works perfectly, but jade complains:

[xref to title unsupported]

This is just an annoyance, since the pdf target is correcly generated, but I would like to get rid of the error messages.
How can I customize the dtd to allow xref to titles? I tried:

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN" "http://www.oasis-open.org/docbook/xml/4.1/docbookx.dtd"[
<!ENTITY % local.title.attrib "xref">

 but that didn't do it.