WIX-based installation results in an invalid configuration file

EDIT

I've decided to try to find a simpler way to ask this question:

If my WIX-based installation needs to modify an XML file that looks like this:

<a>
  <b/>
</a>

into one that looks like this:

<a>
  <c/>
  <b/>
<a/>

Can it be done without a custom action?

Using the XmlConfig element, the closest I can get is this:

<a>
  <b/>
  <c/>
<a/>

The problem, if I have not made it plain, is that the inserted c element must come before the b element already in the file.

XmlConfig and XmlFile get converted to custom actions btw. Can you start with a different xml file such that you can add all children nodes to the parent in the correct sequence? Otherwise, you can request this behavior as a new feature request to the WiX team here: http://sourceforge.net/tracker/?group_id=105970&atid=642717