I’ve been working on a very similar framework called Joose, that some some very similar things in slightly different ways (For some example code: http://code.google.com/p/joose-js/wiki/CookbookRecipe2)

Joose has the concept of roles (a mixture of mixins and java interfaces) which allow for some really nice extention mechanims. Especially if you define method modifiers like before and after inside the role which get applied to a using class at composition time. This allows for some very interesting plugin mechanisms where its much harder to treat on own’s others toes.