The most useful thing, in my view, is a BCP on how MUAs should
generate messages with BCC recipients, and how MUAs should recognize
when a received message is a result of a BCC, and how to handle
replies etc. in such cases.
I agree that this is more important than having an extension to offload
Bcc: processing to the MSA.
I'm intrigued by the behavior mentioned earlier of the originating
MUA mucking with the To and Cc header fields when sending a BCC
version, as an indication of this and to avoid unthinking
reply-to-all actions. I'm not sure I'm a huge fan of the "[To:]"
business, but enclosing the contents in parens to form a comment
might work.
I don't think that just having a comment is valid by RFC 5322 rules. That
could be an issue.
Ned