I think I'd like to see this functionality in trunk, too. Previously
when I needed a M2M model with sequence or other data, I just created
the M2M model explicitly with ForeignKey fields to the two related
models.

    class Role(models.Model):
        role = models.CharField(maxlength=255)

    class Film(models.Model):
        title = models.CharField(maxlength=255)

    class Actor(models.Model):
        name = models.CharField(maxlength=255)

    class ActorFilmRole(models.Model):
        actor = models.ForeignKey(Actor)
        film = models.ForeignKey(Film)
        role = models.ForeignKey(Role)
        sequence = models.IntegerField()

This seems to work, but is it missing out on some of the niceties /
helper methods of the proposed M2M? What would be the benefits of the
suggested new method over this?