I see too many issues with this (for example):
- I publish a news article. I publish a retraction/update to said article. Now the article has a new hash. Does the old hash give you the old version of the article, or redirect you to the new version?
- How do we define 'document?' If we define it as the complete HTML page served up to the browser, then changes to the design of the site would invalidate all previous hashes. Pointing old hashes to new hashes is work, which will not always be done (leading to the same situation we have with site redesigns breaking old URLs).