There's a tool called git-fat, which is used to store large binary files separate from the Git repository. This reduces the size of Git repositories (since they don't handle large binary files very well).
This tool uses rsync, but Phabricator's file storage is a good candidate, especially since it can plug into various different backends (MySQL, S3, etc)
I'm imagining that Arcanist would automatically set up the appropriate Git hooks when it detects a repository using this functionality.