> I know that symbolic links are preferred, but they won't work (if I
> understand this correctly) for distinct, remote-mounted rootfs's

If you can use a hard link, both directory entries live in the same
filesystem.  A relative symlink will always work in that case.

> and hard linking each file seems an absurd waste of i-nodes.

A hard link doesn't consume any inodes; it's just a directory entry.
Hard-linking each file will "waste" one i-node per directory, not one
per file.

So you have two better alternatives to hard-linking directories, a
concept which the filesystem cannot support and still maintain safety
and consistency.