If a target class has getHandler, Javassist does not add getHandler(). So proxifying that class does not cause any problem. On the other hand, since setHandler added by Javassist takes a javassist.util.proxy.MethodHandler object, it should be able to coexist with other setHandler methods, which take different type of parameters. I mean, it would safely overload other setHandler methods.
Sorry, this workaround is slightly complex but I hope the description above makes the trick clear.