To git-annex-matching-options, add --gitattribute option analogous to the current --metadata option but reading a value from .gitattributes.
Unclear what to do when different repo paths with conflicting .gitattributes point to the same content, but that can already happen with --include=glob/--exclude=glob? How is it handled there?
I'm not super enthused by this idea. The git-check-attr interface would make it hard to avoid using one process per use of the option. Checking attributes can be slow. Attributes are limited in what they can contain. Some places where git-annex uses gitattributes I have ended up regretting it. A compelling use case would be good.
Might also be possible to support this in preferred content. Although it looks a bit hard to do, because it would need to start up a git-check-attr process for each different attribute used, while parsing the preferred content expression.