What I would like is an app that I can let loose on my Maildir directory and indexes and allows search. It doesn't need to receive mails or download (or send) them.

You can check out https://github.com/rustmailer/bichon/wiki/Using-Bichonctl-Fo... — it explains how to use bichonctl to batch-import .eml files into Bichon. This makes it easy to ingest Maildir or any folder of EMLs for indexing and search.