We need a good API to export metadata of index pages in a standardized and interoperable format in order to share them with GLAM community. They use the OAI-MPH protocol http://www.openarchives.org/OAI/openarchivesprotocol.html . I think that it may be a good idea to add this feature to Proofreadpage. I'm writing a first version of this feature (demo on labs : http://wikisource-dev.wmflabs.org ).
To add this feature I've written a small metadata typing system. It's a very big addition to ProofreadPage and it need to rewrite the configuration of index pages (bug 37419) in all Wikisources. I suggest to test this big feature in one Wikisource during the needed time of stabilisation before a deployment on all Wikisources when a Special page to help to configure pfp will be done (bug 37839).