Session playback seems like tricky thing. I work a lot with a parametric modeling program (still in Beta) called GenerativeComponents by Bentley Systems. Instead of having a compiled model file at the end of the session, everything is saved as a text file. Each time a project file is opened in the application, it is read by the application and rebuilt in the Model view as well as in a relational graph. Overall the application uses a hybrid of scripting and direct manipulation techniques to create parametric dependencies. The results can be remarkable but it does require the user to be incredibly rigorous along the way with management of the model's elements...sort of the opposite of Moi.