Take a look at these:
https://tools.ietf.org/html/draft-andersen-arc-00 https://tools.ietf.org/html/draft-jones-arc-usage-00
These add new headers based on DKIM that document a chain of custody for mail. It would let mail providers accept list mail that DMARC would otherwise reject. The drafts are a mess (the XML versions are easier to read than the txt), but I think the design is sound. They are intended to be implemented via upgraded DKIM libraries, and the idea is to allow lists to work like they always have, without any changes visible to users. You can intuit from the authors who's likely to implement it.
It'd be helpful for people to look at it and write to the contact address in the first draft and tell them how likely it is that Mailman would support it, or what problems it would present.
R's, John