Modular olinks

["Robert Hall" <rhall@vocera.com>]

I'm trying to implement a free form collection of topic modules as
described in Bob Stayton's book[1].
I put an entity in the targetdoc attribute of the olink element, but I
don't understand where to define the entity value or what syntax to use.
I've tried defining the value in the book file that xincludes the topic
file, and also in the olink database file for the book. For syntax, I've
tried 
<!ENTITY % config SYSTEM "foo"> 
and variations thereof, omitting the % sign and the SYSTEM keyword, etc.
Could someone please give me a gentle shove in the right direction?
Below are some (simplified) sample snippets.
Thanks in advance,
-Robert

Topic.xml (resides in \bar)
--------------------------
See <olink targetdoc="&config;" targetptr="OtherTopic"/> for details. 


Foo_Book.xml (resides in \foo)
------------------------------
<!-- I tried defining the config entity in this file -->
<xi:include href="../bar/Topic.xml" 
            xmlns:xi="http://www.w3.org/2001/XInclude"; /> 


Olink.db (resides in \foo)
-------------------------
<!-- I tried defining the config entity in this file -->
<sitemap>
  <dir name="foo">
    <document targetdoc="foo">
      &foo_targets;
    </document>
  </dir>
					
  <dir name="bar">
    <document targetdoc="bar"> 
      &bar_targets;
    </document>
  </dir>					
</sitemap>

[1] http://www.sagehill.net/docbookxsl/modular-olinks.html