Customizing cross-references for unnumbered <sect3>s

I have my <sect3>s and sections below unnumbered, by customizing the 
stylesheets:

     <l:context name="title-numbered">
       <l:template name="sect3" text="%t"/>
       <l:template name="sect4" text="%t"/>
       <l:template name="sect5" text="%t"/>
     </l:context>

Now when I make an xref to a sect3, I want to generate the number of the 
*parent* sect2 of this sect3 (because the sect3 has no number). (The 
stylesheets generate the complete number of the sect3).

How do I do this?

Greetings,
Joachim
-- 
Joachim Ziegler                       Stuhlsatzenhausweg 85
Max-Planck-Institut für Informatik    66123 Saarbrücken
Email:           Germany