As we discussed in IRC, I think the ability to inject topic titles and text would be useful. The use I have in mind is building up lists of terms and definitions, but it could also be useful when having the concept inline really clarifies the task, but we don't want to duplicate content. Perhaps we could use roles on items within titles to exclude them from the insertion. So here is some XML to illustrate these ideas:

Example of a list of terms:
<section>
  <title>List of terms</title>
  <variablelist>
	<varlistentry>
	  <term><!-- InjectTitle: 1234 --></term>
	  <listitem>
		<para>
		  <!-- InjectText: 1234 -->
		</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><!-- InjectTitle: 2345 --></term>
	  <listitem>
		<para>
		  <!-- InjectText: 2345 -->
		</para>
	  </listitem>
	</varlistentry>
  </variablelist>
</section>

Example of a topic with excluded text:
<section>
  <title>Concept with excluded text</title>
  <para>
	A concept with excluded text is a concept where you don't want all of the
	text to be included in a text injection.
  </para>
  <para role="noinject">
	This paragraph would not be injected if I asked to inject the text of this
	topic.
  </para>
  <para>
	This one would be injected right after the first one. This might require
	modifications to the DTD and I am not sure if that is cool or not.
  </para>
</section>