Some people have been asking this forever, but because it wasn't possible to implement it as a plugin, I hadn't done it. Now, due to some API changes for other purposes, it was suddenly easy.

So, if you want to try it, grab the latest CVS code / nightly CVS snapshot and http://dovecot.org/patches/1.0/zlib-plugin.tar.gz

Compilation instructions inside zlib-plugin.c file.

What it does is check if mbox name ends with ".gz", if so it opens it as read-only using zlib. If the file isn't actually zlib compressed, zlib seems to be able to still access it as a normal file (but it's still read-only).

It should be possible to implement it for maildir files as well by hooking into mail_get_stream().