I have been trying to do the same myself. What I found was that the "Export Ant Buildfile" gets kicked off in the org.eclipse.ant.internal.ui.datatransfer.AntBuildfileExportPage.java file. This resides in the org.eclipse.ant.ui plugin.
To view the source, use the Plug-in Development perspective and open the Plug-ins view. Then right-click on the org.eclipse.ant.ui plugin and select import as > source project.
My plan is to create a Java program to programmatically kick off the ant buildfile generation and call this in an Ant file every time I build by adding the ant file to the builders of my projects (Right-click preferences on a projet, under the builders tab).