Hello Christian, unfortunately no, I decided to leave it as is and I have frozen the Hibernate version of my app to the last compatible version.
There are alternatives which I have not tried yet, that involve manipulating a shared (application scope) or server side scope library in Websphere. So you can define prior to deployment, a shared library with the latest hibernate and javassist. Then associate this library ti your application. Then you can play around with the PARENT_LAST (again prior to deployment) setting of your application, so that your module is picking up classpath from bottom to top, the shared lib java assist is going to be picked first and it is supposed to work.
Dont ask me if I tried it no, websphere configuration and classloader settings through their tools and scripting engine is a mess. : /