We have templates available (for customers, but I can mail them to you) which generate a common base class between the generated entities and the central base class EntityBase2. You could add to that generated class the override of that OnPropertyChanged method so it works for all entities.
You can also use a different route:
Create a partial class of DataAccessAdapter in the dbspecific project and override OnBeforeEntitySave and set the field there. This then works for all entities as well.
In there do something like:
entitySaved.SetNewFieldValue("ModifiedDate", DateTime.Now);
it then will be included in the query when the query is generated