I don't think it's possible for hard links to span filesystems. The point of a hard link is
that the two names have the same inode, and a dentry pointing to an inode on a different
filesystem either wouldn't work at all (since the dentry doesn't include enough information to
specify the inode completely) or wouldn't be really a separate filesystem.
I suppose, however, you could have a single filesystem that you'd carved up and arranged with
bind mounts to look like multiple filesystems, and the OS wouldn't necessarily block creating
the hard links (although, IIRC, Linux presently does).