I've collected all of the DMARC workarounds I know on the ASRG wiki:
http://wiki.asrg.sp.am/wiki/Mitigating_DMARC_damage_to_third_party_mail
Two responses to that, in no particular order of importance:
1. So you said, and yet the mere existence of that page out on the
intertubez has (oddly enough) not yet spurred the secretariat into action.
The big change with DMARC is a deprecation of the Sender/From differentiation,
effectively requiring that these two will be the same. It seems that big
systems have voted that the differentiation causes more harm (spam, phish) than
good (remailers).
Of the responses listed, the one that clearly works is to ask forwarders to
forward messages, what the wiki calls "message wrapping." It works in the sense
that the mail system sees consistent headers that pass all verifications, and
represent the actual action of the remailer while not relying on Sender/From
differences.
At that point, the issue is mostly with the UI. If my reader did recognize the
"simple forwarding" case from "authorized remailers," then the message wrapping
solution would be just fine. The good thing is that it is very much under my
control.
-- Christian Huitema