it turns out that API:Imageinfo does offer image license info with iiprop=extmetadata, e.g.
File:Gull portrait ca usa.jpg: https://commons.wikimedia.org/w/api.php?action=query&titles=File:Gull%20portrait%20ca%20usa.jpg&prop=imageinfo&iiprop=metadata|commonmetadata|extmetadata - AttributionRequired and Credit entries are especially useful, so it may be worth mentioning them
