question came up on neurostars and I had no good answer: what would be the best filesystem to use on external drives which would be portable (mountable) across OSes and would provide git-annex with needed features (symlinks, permissions handling, and locking?).

I love BTRFS on Linux but I do not think it is (and neither ext* family) is portable. Is there any filesystem which would be portable across Windows/OSX/Windows and still work with git-annex without needing adjusted-branches mode?