I condsider it completely unacceptable to expect a message header field
to modify envelope information, with or without an SMTP extension.
Furthermore, since alias expansion or forwarding can be done at any point
in the signal path, I don't see any way of implementing this functionality
that doesn't present a risk that an excluded recipient can learn that he's
being excluded. (I note that there's no security analysis in this document.)
More generally, I'm not sure that this is desirable in general. If a
particular list wants to provide people with an option to mail to the list
while still excluding some people (and I have occasionally seen a need to
do this) then perhaps the list could provide a web interface for that rare
purpose. I don't think it's worth defining an SMTP extension.