An ability to copy key availability information from one git-annex'ed repo to another had been a use case we needed for quite some time. Michael even implemented datalad copy-file remotes - that was an immediate use-case whenever it was necessary to craft "custom" smaller datasets from a much larger (super)dataset . And now we came to the same need to facilitate "splitting" a larger dataset into sub-datasets: issues/600.

I think, in particular with, something like git annex copy-key --from hidden-repo KEY could become the ultimate tool for such operations. Alternatively may be it could be git annex copy-key --from-repo /repo/path KEY so there would not even be a need to link original one as some hidden repo, but rather even "include" original one as a git submodule (e.g. original/).

Such command would need to pretty much do whatever git annex merge already does, just limiting its effects to only relevant key(s) and only the remotes which are known to have that key.

Although I concentrated on "copy-key" pretty much the same feature could be useful to provide a git annex copy-file PATH1 PATH2 ... DEST where PATH?s would be paths in other git-annex repo(s) -- so it would pretty be similar (just better since not only URLs copied) to datalad copy-file.

--batch mode in both cases would be super handy to streamline use of the command(s).

WDYT Joey?