Edit your config.xml to include something like this:
<schedule interval="3600">
<composite>
<ant antworkingdir="checkout/${project.name}"
buildfile="build.xml" target="izpack" />
<exec workingdir="checkout/${project.name}/dist"
command="/usr/bin/python"
args="/Applications/IzPack/utils/izpack2app/izpack2app.py
installer.jar installer.app"/>
<exec workingdir="checkout/${project.name}/dist"
command="/usr/bin/tar" args="cvfz installer.zip installer.app" />
<exec workingdir="checkout/${project.name}/dist"
command="/usr/bin/python"
args="/Applications/IzPack/utils/izpack2exe/izpack2exe.py
--file=installer.jar --output=installer.exe --no-upx />
</composite>
</schedule>
The four steps are compile from your build.xml, giving your installer.jar. Create the Mac app, zip it, then create the Windows executable. You need 7Zip for the Windows part, Mac users get it via MacPorts: sudo port install p7zip. Don't forget to add the new files to your artifactspublisher.