> In the case of write access, you only need back-pointers (from inode to directory) like btrfs has to trivially get "_all_ the directory entries referring to that file".
You'd still need write access to all those directories to change the permissions stored in the directory entries. That can be a problem if you own the file, but someone else owns some or all of those directories. Alternatively, the system could grant write access to directory entries based on the ownership of the target inode rather than the ownership of the directory. Both methods are counter-intuitive.
Regular POSIX ACLs can already express the union of two sets of permissions. There is no need to vary the permissions based on the path used to access the file.