Another possible solution would be to ommit the code sections that do the casting at all and replace them with:
http://www.csg.ci.i.u-tokyo.ac.jp/~chiba/javassist/html/javassist/util/proxy/ProxyFactory.html#create%28java.lang.Class[],%20java.lang.Object[],%20javassist.util.proxy.MethodHandler%29

Since this method was added in 3.4 we could make hibernate backwards compatible with even less effort.