Apology for the tardy response – I missed your question. I’m not familiar with a specific tool in the WordPress environment that solves the challenge. The only related technology I can think of is the OpenAttribute (browser plugin) http://openattribute.com/ which helps on the authoring side.
In the Mediawiki environment we keep the metadata and licensing with the image page and when using the wiki –> print technology, attributions are handled automatically. In theory it should be possible to replicate that for Pressbooks?