It would be good if one could define custom external [[backends]], the way one can define external custom remotes. This would solve [[todo/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.
