I have no idea of python-coding, but I thought for some experienced person here it should be no problem to add the feature to mailman that it writes also to sendmails' aliases and virtuser-files when setting up a mailing list.

See README.SENDMAIL and the contrib/ files it refers to. (This is in MM 2.1, of course).

I don't use it, but I'm sure David does, and I'm sure it works.