Malachi you are right, I was too quick with my answer.
After checking again, I managed to make the Jason plugin work with the following changes to its ant script:
...
<property name="jaxb.home" value="/path/to/jaxbEA3" />
<property name="jars" value="." />
...
...
<arg line="-debug -nv -extension -Xlocator -Xhibernate-java" />
...