But a serialized model should be materializable in, for instance, an UI-less server, or some client with a very different UI framework, etc. This would enable (among many other useful things) sharing serialized objects between Cuis, Squeak and Pharo.

Sorry, let me see if I understood: You mean that as the model has references to UI objects but they are just weak references, then "Serializer serialize: model" should not include UI objects and thus you can transport the model with UI-framework independence. Am I right?