This is a patched version of GNU Mailman which integrates support for searching list archives using htdig. This is integrated in the Wikimedia Mailman .deb.
Apply these patches to the Mailman source before installation:
The patch documentation describes how to enable it; this is not enabled in the default Mailman configuration file in the package.
htdig can be installed via yum on fedora and APT on Ubuntu.
Fun warning! Search result pages will come up blank with no error messages if htsearch is unable to locate the 'COMMON_DIR' which contains its HTML templates and images.
For some reason, the fedora package installs this directory at /usr/share/htdig but the binaries look for it at /var/www/html/htdig. I made a manual symlink to that disused location.
Additionally, that directory should be aliased in to /htdig on the mail web server so that the icons show up in result pages; for the current setup I just symlinked it in from the docroot.
Search indexes are updated by a nightly cronjob, at /var/lib/mailman/cron/nightly_htdig which is called from /etc/cron.d/mailman.
Initially, lists are set up for htdig as they first receive a mail after the configuration is enabled; but they won't be indexed until the cronjob next runs. (It can also be manually run, of course.)