A "difference=" parameter was added to multipart/alternative in RFC
1766 (content-language), to facilitate easier parsing of messages that
had the same content in two different languages.
This is currently at Proposed, while multipart/alternative is at Draft;
if someone implements it and declares it good, it will probably be
integrated with the MIME spec once they somehow reach the same level
of maturity.
(Yes, it's little and late. Only experience will tell us if it is better
than nothing.)
Harald A