Hi Magnus, I found one of your old tweets regarding search accorss all wikis at same time.
If you have not yet found a solution, consider looking at wikitech:Help:CirrusSearch_elasticsearch_replicas. Full replica of the "search" backend used by mediawiki, as direct access using Elasticsearch http api.
Here is exemple with pywikibot. Just simple http unauthenticated queries 
For your specific use-case (search on titles across all wikis), look at this repo that does same, you'll have to filter on the "title" field.
Thanks for your work!!!