Apparently, there's a poorly-documented feature of Mailman that gets you the complete list history in mbox format. See here. This is nice because email addresses and dates and headers and attachments are unmangled.
I used this on the Flashrom mailing list and got the complete history from this address, http://www.flashrom.org/mailman/private/flashrom.mbox/flashrom.mbox, for example. For that list, at least, you have to be a subscriber and logged in via the web interface in order to download the mbox file. In my opinion, that's a reasonable anti-spam measure, whereas mangling everything that looks like an email address is not.