On using the cache for tracking, I don't think that'd be an issue with what I'm thinking about here because all the libraries in this index would be pre-populat

On using the cache for tracking, I don't think that'd be an issue with what I'm thinking about here because all the libraries in this index would be pre-populated in this special browser 'cache' (maybe cache is a misleading term here, store or repository might be a better one). Or said another way, there'd be no information to be gained from combinations of cache hits & misses because there would only ever be hits.