class FooBaseModel(models.Model):
    bar = models.CharField(help_text=_("before"))
    class Meta:
        abstract = True

class FooBar(FooBaseModel):
    def __init__(self, *args, **kwargs):
        super(FooBar, self).__init__(*args, **kwargs)

        # Change field data in a DRY way
        bar_field = self._meta.get_field_by_name("bar")[0]
        bar_field.help_text = _("New help text!")