I recently implemented functionality to correct problems like this. My changes have been accepted and placed into CVS. You can check out the latest from CVS and use this new functionality.
I described how to use the new functionality in http://forum.hibernate.org/viewtopic.php?t=925700&highlight=
. See the message about JavaTypeMapper. The sample file was also placed in CVS under the file HibernateJavaTypeMapper.java in the plugins/hibernate dir. Just point to this file in your hibernate ant task.