I should probably report the experience of mail-archive.com, which has supported a homebrew message-id hash algorithm for over 6 years.

Only one organization has ever used it (LibreOffice). They put it in the Archived-At headers, not message footers. However, links did end up getting manually referenced in message bodies 575 times. Folks here might want to take a look at how people used it. Christian Lohmaier from LibreOffice is a good contact if anyone wants to talk to him directly.

The homebrew has algorithm turned out to be a pretty bad design. Base-64 got us in trouble. One memorable bug involved hashes prepended with a minus sign. This caused havoc with the search engine resolving the links, who interpreted the minus sign as a negation operator.

Blending the List-Post address into the hash was supposed to resolve crossposts. In practice it complicated things enough that LibreOffice incorrectly computed some of the hashes. There were also times when mail-archive.com wasn't doing that great a job either of running everything smoothly.

Finally, we really get in trouble when asked to merge multiple archives, due to a mailing list migrating from server to server. It happens, and those migrations often change both List-Post and List-Id. Blending either into the hash is a headache.

So the homebrew hash was dumb idea, and will gracefully retire in favor of the standard mailman hash algorithm. It was pretty neat to see some usage, though. And Archived-At still looks like a winner.

Cheers, Jeff

Alogithm: https://www.mail-archive.com/faq.html#listserver

Usage: https://www.mail-archive.com/search?l=all&q=go.mail-archive.com