At this year's code4lib, I participated in some discussions about using API-based authority data in VuFind. I've just put together a quick test-of-concept to demonstrate how this could work. As of r2314, I have added a WorldCatTerms recommendations module which uses the WorldCat Terminologies API to suggest broader, narrower and related terms based on a user's search. To test this out, just load the latest SVN code, uncomment the "Subjects = WorldCatTerms" line in web/conf/searches.ini, and do a subject search.
There's obviously room for refinement here, but I was fairly impressed by how even this simple code can offer useful suggestions. I'd be interested to hear what others think.
I should also mention that the basic concept here was inspired by Eric Lease Morgan's TermFinder demo (http://zoia.library.nd.edu/sandbox/term-finder/). What I ended up with is significantly different from his work, but the initial API calls are the same, and I owe him thanks for providing the sample code that I used as a seed.