The proyect we use has a class that implements IDesignTimeResourceWriter that has the method AddResource(string name , object value) that implements System.Resources.IResourceWriter.AddResource, that save the data in a ListDictionary used, some steps later, to create the resources in the DB. This method is called by VS, which send the resource name (e.g. Label1.Text) and the value, and I am again at the beginning.
There is another method System.Web.UI.Design.IDesignTimeResourceWriter.CreateResourceKey, as far I have tested, it is not called anytime.
Thank you, your indications has given me where to look for.