I also have a similar need but for upvoted content. Sometimes I keep scrolling through pages of upvotes to find a specific item. The problem is that favourites and upvotes are not exposed via the API and require scraping, otherwise it would be easier to build a search tool for them.
We'd have to build that into HN itself, since upvote data is private (extremely so). But it's the sort of feature whose value grows as the site continues, so I could imagine us adding it eventually.
