It does cut it down a bit. And doing it that way would certainly make it very "long term survivable". If we put a hash in there, we have to store the hash somewhere. Not that this is hard, but if there is concern about long-termability.
Another option would be to teach the URL shortener about our messages specifically, and just use an internal surrogate key we already have for them. That way it could be something like the current Planet URLs (which can be for example http://postgr.es/p/3Dl), so much shorter. To post one people would have to go to a webform and paste in the messageid or existing URL though -- or we could put a link on the page in the archives.
It would make the URLs actually short, but as mentioned upthread, that wouldn't work at all if offline. So it'd be a tradeoff between those, but so are pretty much all other options that don't include the full message-id.
