(D) Another sneakier idea that came to mind—which may not be technically plausible—would be to find good results in another language and then check for links back to wiki articles in the wiki the search came from.

I don't know if it's technically plausible but AFAIK we have the wikibase id in the index so it's should be pretty simple to extract it. Interwiki links are stored in wikidata, could we use WDQS for that purpose? With the entity ID it should be easy to request the interwiki link for a specific language. Is WDQS designed for this usage (high number of query/sec on rather simple queries)?

 

I also thought of WDQS for this. We should ask Stas.

 

What do you mean by "interwiki link" and from where would we be requesting it? from Cirrus?  interwiki links are in parser output of all client wikis. (e.g. wikipedias)

otherwise the wbgetentities api module in Wikidata has them also, and can't see volume of requests being a problem there.