background image on the titlepage

Hi folks!

I am writing in  Docbook XML compiling documents with
XSLTPROC and FOP 0.94. 

I would like to customize the titlepage by adding a
background image to it. The definition of the
titlepage is located in the pagesetup.xsl

http://docbook.xml-doc.org/snapshots/xsl/fo/pagesetup.xsl

The problem with pagesetup.xsl is that when I only
need to customize, let's say, the titlepage-first 
master page, I need to copy the whole set of master
pages to my customization layer, and that's a huge
amount of redundant code. Otherwise, compilation
fails. Could you please explain how to customize only
the titlepage-first  master page within the set?

Big thanks!  :-)

Nancy