I'd like to do something like git-annex-addurl, except the URL is not a web address but a domain-specific identifier. Specifically, on the DNAnexus platform, each file is assigned an immutable ID like file-asdf432fdsa. I want to configure git-annex so that it knows it can fetch the file using a specific command (not curl but 'dx download file-asdf432fdsa'); and so that it completely trusts that the file can always be fetched this way. What would be the right combination of configurations/settings to make this work? I was thinking of some combination of WORM backend configured just for DNAnexus files and a special remote, but not sure (this "special remote" would only support copying files from it, not to it; so it's more like "special web"). Thanks for any suggestions!