I'd love an extension which adds customized search expressions (sql or otherwise) to be able to search messages in a more advanced way (e.g, messages after date X, with recepient y, not recepient z)
Have you looked at the FiltaQuilla extension? It now has the capability to add a search term which consists of arbitrary javascript. The example that you gave would be fairly easy to implement in a few lines of javascript in FiltaQuilla.
The custom search terms used in FiltaQuilla use the "traditional" search infrastructure, based on per-folder databases, rather than the gloda sql-based method. I suppose what you are really looking for is a search interface to the gloda database, beyond the "search all messages" faceted search that is offerred in TB3. Hopefully asuth (the gloda developer) is working on that.