Where did you get the idea that Mail-Followup-To communicates anything
other than this?
your web page on the subject talks about setting MFT automatically, in
order to suppress duplicate replies. and that seems to be by far the
most common use of MFT.
I also doubt that it's reasonable to use a single header field to
communicate preferences for group replies, because the recipient may
need to act differently depending on _why_ that field was set, and the
recipient's UA may need to act differently (say to alert the recipient
to an exceptional condition) depending on why that field was set. But
I'm still thinking about this.
Keith