There already is one in the current release.
It doesn't directy mess with mailing lists. That is mainly because I
don't know how many people run mailing lists off there web server. I
know we don't and I know I wouldn't suggest anyone do such a thing.
There is nothing wrong with your approch except you have tied it
preaty tightly to what ever release of the code it works against
(ie. the locking has to be the same, You have to read and understand
the config files.)
Other things, I would guess and auto list is the only one that you
would accualy subscribe want to do a subscribe to. Since Closed lists
you always forward anyway. And Open lists you only allow subscritions
from the same name as is sending the mail (you have no idea what the
username is so it fails this test.)