Apple's HFS+ supports multiple forks such as the old data and resource forks. NTFS supports alternate data streams. I believe some *nix filesystems also have some support for multiple file forks or streams.

Given that directories (folders) are just a kind of file at the filesystem level, I'm wondering if any of the filesystems which support this feature support it for dirs as well as files? (Or indeed directories in the alternate forks / streams?)

I'm mostly asking out of curiosity rather than wanting to use such a feature. But one use it would have would be additional metadata for directories, which seems to be the most common use for these streams for files currently.