Just point to the content:

<doc xmlns:xi="http://www.w3.org/2001/XInclude">
<title>My Document</title>
<xi:include href='abstract.xml'/>
</doc>

'abstract.xml' contains:

<abstract>
<p>Blah, blah, blah,...</p>
</abstract>

So you get:

<doc xmlns:xi="http://www.w3.org/2001/XInclude">
<title>My Document</title>
<abstract>
<p>Blah, blah, blah,...</p>
</abstract>
</doc>