I think space savings would be better done by supporting copy-on-write semantics within the file system, which is just what you describe as Immutable-unlink. I imagine COW should be less error-prone than hard links, but just like you I'm no fs guru (or I wouldn't be asking these questions :-).
Something along these lines have been implemented at http://www.ext3cow.com/ (which is a random project I found on Google, I don't know anything about it except what the web page says).
So do you (or anybody else for that part) know of any other uses for hard symlinks that don't have anything to do with space savings?
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