Florian Weimer writes:
Based on some reports, I don't think this is how the Gmail implementation works. It will discard mailing list mail for senders with a DMARC policy that does not set p=reject, too.
Based on conversations with GMail developers, that has nothing to do with DMARC, though. A proper mailing list *also* signs its messages, and it develops a reputation much faster than any individual poster. (It's also more dangerous than individual posters, so that may or may not balance out exactly.) As far as GMail is concerned, therefore, a post by such a user has a valid signature, but the treatment of the post depends only on the reputation of the mailing list and the content of the post, rather than on the reputations of the poster and the list, and the content. Note that this is the same distinction that occurs when the list munges From.
Bottom line: I don't think a per-subscriber option is likely to help subscribers who are GMail users (in general, if you don't own your own MX), and for reasons I've given elsewhere, I think providing a per-subscriber option is likely to be a PITA for list admins (== my primary constituency as a GNU Mailman developer).
Steve