Copy one value over to another with xmltask

Is it possible for an xml task to copy the value of one node into another node for each element in a list?

Source XML:

<a>
 <b>
   <c1>foo</c1>
   <c2></c2>
 </b>
 <b>
   <c1>bar</c1>
   <c2></c2>
 </b>
 ...
</a>

Destination XML:

<a>
 <b>
   <c1>foo</c1>
   <c2>foo</c2>
 </b>
 <b>
   <c1>bar</c1>
   <c2>bar</c2>
 </b>
 ...
</a>

I'm trying to accomplish the above in my ant task, but I con't seem to find a way to do it, here is what I am doing so far,

<target name="mergefile">       
    <!-- Read the source into a buffer -->
    <xmltask source="source.xml" clearBuffers="list"> 
        <copy path="/a" buffer="list" append="true"/> 
    </xmltask>

    <!-- Write them to the output -->
    <xmltask source="destination.xml" dest="destination.xml"
        outputter="simple">
        <!-- First clear all the old paths. -->
        <remove path="/a/b"/> 
    <!-- Then add the resolved ones. -->
        <paste path="/a" buffer="list"></paste>
            <!-- Copy the value over? -->
        <replace path="a/b/c2/text()" withText="No Idea"/>
    </xmltask>
</target>

Any idea of how to copy the value from one node to the next for all the elements in the list?

As, I guess, is usually the case, writing my own task was the only way I could see to do it.

@Override
public void execute() throws BuildException {
   //Read file line by line, regex test on each line, 
   //matches get written back twice.
}

Then calling it was,

<copyregmatch file="myfile.xml" regex=".*replace.*" />