Thanks for the reminder, just voted and commented.
I supported also the OAI-PMH proposal,
but I think it's not the solution, as we already have such a tool and nobody uses it...
It's years (for me, an entire decade...) that we advocate and propose solutions for metadata workflow in Wikisource, and as far as I understand, a big part of it is Wikidata.
IMHO, we should try to put all our descriptive metadata (author, title, etc.) there, with a good, meaningful structure (we should understand which metadata are from the ns0 page, which from the Index pages, they are often different). Then, we could build APIs or other tools on that.
WS search tool was a step in the right direction, but it's a difficult job and it should be a community effort.