we have thought a while about it and needed a solution badly. We are having several Mail2News mailing lists, which all come up with broken threading, when cross posted messages appear. The same issues as stated in the bug here: https://bugs.launchpad.net/mailman/+bug/266263
I have just added a patch and a description for the patch at launchpad.
This is a request for considering this patch as improvement and merging it. A lot of mail and news administrators would be thankful for less headaches :)
kind regards, kloschi