this is not really solveable by wicket itself. We don't know where such a reference is comming from, so this should be documented i guess that if you use session data from the session object itself. You should always use an extra indiretion to get it new Model() { getObject() {Session.get().getList()} }
I don't see a generic solution either, but in this case I though you should use custom serialization of either your custom session or a model you use for the extra indirection. Eelco