In our environment with IBM WAS 8.5, we get the following exception upon activating the hibernate.ejb.use_class_enhancer option:

[5/12/14 15:23:16:774 CEST] 00000034 JPAPUnitInfo  E   CWWJP0014E: An illegal format has been detected during the class transformation for the ...xxx... class. The class has not been transformed.
[5/12/14 15:23:16:787 CEST] 00000034 JavassistClas Z org.hibernate.bytecode.internal.javassist.JavassistClassTransformer doTransform HHH000373: Unable to transform class: cannot find ...xxx...

This is due to a missing classpath in the created classpool in class JavassistClassTransformer:

ClassPool cp = new ClassPool();
cp.appendSystemPath();
cp.appendClassPath(new ClassClassPath(this.getClass()));
cp.appendClassPath(new ClassClassPath(classfile.getClass()));
// the following class path is new and includes the passed classloader from the method parameters
cp.appendClassPath(new LoaderClassPath(loader));

The problem occurs because we have hibernate defined in a shared library and not included directly in the application itself. Thus it's not possible through the hibernate classloader to find the needed application classes. With the aforementioned change, the runtime weaving is working as expected.