Is it possible to write node migrations for D8 alpha/nightly yet? If the migrate api in core is currently capable of doing simple d8-to-d8 migrations, where could I find documentation on how to do so?

This is more or less a learning exercise and I am only trying to migrate standard article nodes from one alpha version to the next.
We haven't written Drupal 8 sources yet and they won't be written until Drupal 6 and Drupal 7 is done. It's not a priority. You could, of course, help with writing them :)