i don't think you can output a file with SchemaUpdate, but you can with SchemaExport.
this is from the online documentation:
http://www.hibernate.org/hib_docs/v3/re ... guide-s1-4
20.1.4. Using Ant
You can call SchemaExport from your Ant build script:
<target name="schemaexport">
<taskdef name="schemaexport"
classname="org.hibernate.tool.hbm2ddl.SchemaExportTask"
classpathref="class.path"/>
<schemaexport
properties="hibernate.properties"
quiet="no"
text="no"
drop="no"
delimiter=";"
output="schema-export.sql">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaexport>
</target>