Have been working on a design for exporting trees to special remotes. As well as being handy for publishing scientific data sets out of git-annex repositories, that covers long-requested features like ?dumb, unsafe, human-readable backend.
I had not been optimistic about such requests, which seemed half-baked, but Yoh came up with idea of exporting a git treeish, and remembering the last exported treeish so a subsequent export can be done incrementally, and can fully sync the exported tree.
Please take a look at the design if you've wanted to use git-annex for some sort of tree export before, and see if it meets your needs.