Postorius has CSRF protection so adapting this will be more complicated as you will need to 1) hit the subscription endpoint (which also does the MM2 or MM3 check by checking the response) 2) extract the CSRF token 3) pass back the same cookies to <stuff>/anonymous_subscribe with the token and email address.
It might just be easier to provide a link saying "sign up for mediawiki-announce" but I doubt we'd get the same conversion rate.