I have a ruby script that creates a release branch from trunk at the start of every release cycle in my project. we recently moved to hudson so I though it will be good if the script itself downloads and modifies Hudson release build config and posts it back. I read from here that it is possible to programatically modify the configuration on Hudson and POST the same XML.
Have you tried posting your xml file using some http utility tools like Poster in Firefox to your hudson server ('http://my.buildserver.com/hudson/job/branch/config.xml') ?
Does it accept such submission ?