The problem is that the receiving service will look at who deliveredIt occurs to me that this also applies to normal group postings coming from services that do not require the DMARC mung of their From address.
So maybe that workaround on the outbound would help. That is, for every /receiving/ service that has a high rate of unsubscribes, mung all From addresses sent to that service.
I noticed that it was generally the same services that publish DMARC p=reject that show up as those operating FBLs with the highest perceived rates of member unsubscriptions. Maybe that's not a coincidence, maybe those services are also particularly sensitive to inbound spoofs, even when the sending service does not publish a sequester policy, or has no policy at all.