I have managed to create my own MySQL-based member adaptor and have integrated it into one of my mailing lists using the extend.py mechanism. I was very pleased when it worked with very little debugging, however I found later it is only half working.
When I go to the administrative interface of the list, I can see the members from the MySQL database in the membership list. However, when email is sent to the list from one of the member addresses, it is held for approval due to "post by non-member to member-only list". When approved, the message is added to the archive, but no one in the membership receives it. It's as if Mailman is using my Member Adaptor for some tasks, but using the old method of member storage for the important stuff, e.g. accepting and distributing messages.