Presumably a "smart directory" would itself be read-only, as if mounted from an ROFS; but its contents would all be symbolic links, such that when you're opening the file, you're opening its readlink(2) target, so the fact that the source link was read-only doesn't matter to your ability to modify the file.
This would pretty much match the expectations of how "Smart Folders" currently work in the Finder, and also how equivalents like "Smart Playlists" work in iTunes/Music.
This would pretty much match the expectations of how "Smart Folders" currently work in the Finder, and also how equivalents like "Smart Playlists" work in iTunes/Music.