I mentioned before that my solution to this was to write a
post-processor that fixed up the headers. Sort of like YARF, but
more flexible for a list-owner or moderator. It's automated and
requires very little work. Unfortunately, I wrote it as a quick
hack and never got around to making it more general purpose.
I'm moving to Linux and have discovered Mutt. It's somewhat like
Yarn, but allows editing the headers on the fly (and a bunch of
other stuff). The international version has PGP support. It's free,
under very active development, and it has source code. If any of
you are running *nix, I suggest having a look.
-rex