It would be good if one could define custom external backends, the way one can define external custom remotes. This would solve consider meow backend but also have other uses. For instance, sometimes files contain details irrelevant to the file's semantics (e.g. comments), but that change the file's checksum; with a custom backend, one could "canonicalize" a file before computing the checksum.

@joey pointed out a potential problem: "needing to deal with the backend being missing or failing to work could have wide repurcussions in the code base." I wonder if there are ways around that. Suppose you specified a default backend to use in case a custom one was unavailable? Then you could always compute a key from a file, even if it's not in the right backend. And once a key is stored in git-annex, most of git-annex treats the key as just a string. If the custom backend supports checksum verification, without the backend's implementation, keys from that backend would be treated like WORM/URL keys that do not support checksum checking.

Thoughts?