The good folks at WMDE have been working on a method of searching for and returning sub categories when a user requests it via special search.
We'd like to investigate if using Blazegraph would be a good method of use to implement this in production. It would probably be an extension into CirrusSearch (of some sort) to allow for searching (and returning results from) sub-categories when the main category is searched on, when using the advanced search functionality.
We'd also need to look into how to update the categories on some sort of regular basis, automatically.
This was discussed at the Vienna Hackathon 2017.