I've just found [[Special:ProofreadIndexOai]] and loved it!
Firstly, why this special page isn't listed at [[Special:SpecialPages]]?
Second, [1] points to [2], that returns a 404 error.
Oka, now I'll ask the most important thing: how to implement it!? =)
The doc page [3] mentions a page stored in the MediaWiki namespace, but that page is almost empty on it.wikisource and don't ever was created on en.wikisource. Those domains contains DC fields such as title, creator, issued.
The MediaWiki page was also ever created on pt.wikisource, but all metadata that it returns to me is type and format.
---- As a side note, I really love all efforts done in ProofreadPage extension. It's amazing that a plugin developed entirely in a volunteer basis, with little or none input from the main product (MediaWiki) developer team, has achieved such level of quality and complexity.
But, please, give some love and attention to maintain it's documentation. Do it remembering that it will be crucial for small Wikisources, since most of the time only one or two users retains all the very specific knowledge necessary to try to develop and atract new users for that subdomain.
Reverse engineering to discover how something works is a time consuming task, like all the remaining core tasks. At the end, those local experts users ends in getting no fun contributing to Wikisource because firstly needs to do lots of additional and sometimes bureaucratic work if really wants to not being the only user in such wiki.
We all are in the same boat, trying to colect works in copyleft or public domain in a project that the main organization behind we (Wikimedia Foundation) neither try to help us or know how to help us (in fact, mostly of us, myself included, don't know also on what can be helped), please help each other on what is possible.
[what a melodramatic message I just wrote LOL]
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[1] - https://wikisource.org/wiki/Special:ProofreadIndexOai?verb=ListRecords&m...
[2] - http://mediawiki.org/xml/proofreadpage/qdc/
[3] - https://www.mediawiki.org/wiki/Extension:Proofread_Page#OAI-PMH