I got errors on building dom4j, because of wrong bytecode version of the jaxme library jar:
[javac] bad class file: /var/tmp/portage/dom4j-1.6.1-r1/work/dom4j-1.6.1/lib/jaxmeapi.jar(javax/xml/bind/JAXBContext.class)
Jaxme compiles fine though, btw.
I looked into the jaxme source package, and it seems it uses several build files not called build.xml but build*.xml.
Thus, my guess is, the ebuild needs a JAVA_PKG_BSFIX_NAME="build.xml buildxs.xml", or even some more build*.xml entries.
Works fine now, thank you.
I wonder what it has to do with the upgrade guide though (which I kinda know by heart by now :P)