I have been working on a fix for the problem of Mailman's dropping format=flowed from Content-Type: headers. There is a bug report and a patch at https://sourceforge.net/tracker/?func=detail&atid=100103&aid=1495122&group_id=103.
The patch to Scrubber.py has just been revised (revised patch is attached to the above tracker item).
Scrubber would throw an exception when processing a message with no text/plain part. If you are using a prior version of this patch, please get the current version to avoid this problem.