Version 1.5.1 of xmltask is out now. This version supports a character encoding definition for the output file.
eg. <xmltask encoding="shift-JIS" ...>
See http://www.oopsconsultancy.com/software/xmltask.html
xmltask provides the facility for automatically editing XML files as part of an Ant build. Unlike the standard filter task provided with Ant, it is XML-sensitive, but doesn't require you to define XSLTs.