The link targets of annexed files are currently very long. This creates problems e.g. when browsing directories in Emacs (I mostly work through a text terminal). Ideally, the key would not be repeated twice in the link, but I understand this is hard to do compatibly. Maybe, the following simpler alternative could be implemented? Key checksums are currently represented in base16 using only the characters 0-9a-f . The same information could be represented with shorter strings using base64url or other encoding, where a larger range of chars is used. So for each backend you'd add a corresponding one that does the same thing, but encodes the checksum part of the key with shorter strings.
Or, if you're tired of backend requests, maybe implement a scheme for external backends, like the one for external special remotes? For external backend EXTNNN the user would put a script git-annex-external-backend-NNN in the path; the script would support commands like calckey, examinekey . Then I could also implement e.g. canonicalizing backends that strip away variable but semantically irrelevant information before computing the checksum.
