Hello,
I use hibernate tools and ant task like this :
Code:
   <taskdef name="hibernatetool" classname="org.hibernate.tool.ant.HibernateToolTask" classpathref="class.path" />
   <target name="jpa2ddl" depends="init"  description="build ddl from jpa files">
      <hibernatetool destdir="${generated.ddl.dir}">
         <jpaconfiguration/>
         <hbm2ddl drop="true" create="true" export="false" update="false" delimiter=";" format="true" outputfilename="${schema.export.file}" />
      </hibernatetool>
   </target>
Unfortunatly, this task look after a persistence.xml file.
See hibernate tools documentation for more details