Pardon me if I'm getting to be a broken record on this.
It would be very nice to be able to have a pattern that generates
a reference to the archived message based on information that all
recipients from a mailing list will have.
The kneejerk canonical ID for a message is its Message-ID such
as what you build links with.
The scenario I have in which I would like to do this is that
different sites archive the same messages and the archives want
to be able to cross-link between copies of the same message.
For example, I would like to be able to create a topical archive
combining messages that have not all moved through any one list
and have the list-borne messages, as archived, refer to the
archives for their respective lists-of-origin.
An alternative is what the W3 consortium is now doing with its
lists. It assignes an archive message-number as the message
passes through the mailing list manager, so that all recipients
have it when they receive the mailed copy of the message.
If one could define variable definitions like procmail recipes,
going from a regexp which catches header names to a form that
writes the variable value [classic match/mold pair], then this
header could be trapped and a link to
this-message-in-master-archive could be generated.