> Since MASS is not using S/MIME in any way shape or form, I doubt you are
> offending them.
I am offending them in that the way I would solve this problem is by
putting headers in an inner attachment, which you've pointed out (twice)
is a non-starter for them. If we try to come up with a unified mechanism
for doing this, my strawman would be:
1. Inner message/rfc822
2. Rules for merging headers
FWIW, at some of the groundwork for header merging can be found in the
specification for how to handle message/partial in RFC 2046.