Not sure, if this is exactly what you want / if it full fills your requirements, but I'm thinking of ENTITYs. You can define them at the top (of your XML document, so general XML, nothing DocBook specific). As seen here for the 'doc.release.number' and 'doc.release.date'. But they can also be included through an separate file. As seen in the 3th ENTITY row. Here the SYSTEM means, comming from another file 'entities.ent'.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
<!ENTITY doc.release.number "1.0.0.beta-1" >
<!ENTITY doc.release.date "April 2010" >
<!ENTITY % entities SYSTEM "entities.ent" >
%entities;
]>
<!-- This document is based on http://readyset.tigris.org/nonav/templates/userguide.html -->
<article lang="en">
<articleinfo>
<title>&project.impl.title; - User Manual</title>
<subtitle></subtitle>
<date>&project.impl.release.date;</date>
<copyright>
<year>doc.release.year</year>
<holder>Team - &project.impl.title;</holder>
</copyright>
<releaseinfo>&doc.release.number;</releaseinfo>
</articleinfo>
<section>
<title>Introduction</title>
<para>
The &project.impl.title; has been created to clean up (X)HTML and XML documents as part of
</para>
<section>
</article>
In the document you reference the entities through a starting &
and ending ;
as in &project.impl.title;
In the file 'entities.ent' you specify the ENTITY elements in a similar way:
<?xml version="1.0" encoding="UTF-8"?>
<!ENTITY project.impl.title 'Maven Tidy Plug-in' >
<!ENTITY project.impl.group-id 'net.sourceforge.docbook-utils.maven-plugin' >
<!ENTITY project.impl.artifact-id 'maven-tidy-plugin' >
<!ENTITY project.impl.release.number '1.0.0.beta-1' >
<!ENTITY project.impl.release.date 'April 2010' >
<!ENTITY project.impl.release.year '2010' >
<!ENTITY project.impl.url '../' >
<!ENTITY project.spec.title '' >
<!ENTITY project.spec.release.number '' >
<!ENTITY project.spec.release.date '' >
<!ENTITY doc.release.year '2010' >