I would expect that $config['maintain_ratio'] = TRUE;
would cause the width to be calculated if I only specify a height. I guess I have to provide any large number for the height to make it work?
$config['image_library'] = 'gd2';
$config['source_image'] = $image_filepath;
$config['new_image'] = $thumbnail_filepath;
$config['maintain_ratio'] = TRUE;
$config['height'] = $this->thumbnail_height;
$this->image_lib->initialize($config);