This thread is a bit dusty, but I just want to mention, that I made some improvements to andrea's implementation, in case somebody is still interested in interfaces like this, while vtrefs are still unimplemented.
Mainly I fixed it(previously the generated AST was invalid) and added a way to export interfaces. I've already made a [pull request](https://github.com/andreaferretti/interfaced/pull/1).
