It is a long time since I last deluded myself that a hard link to a directory would be a good thing. Even then it was discouraged due to concerns with infinite loops in directory traversal. IMHO it is a good thing that it is more strongly prohibited in most systems these days.
I did come across a simple algorithm for resolving paths to a "canonical" path even if hard links to directories are allowed: follow all the links to the ultimate directory, then follow the chain of ".." back to the root directory. This assumes only that ".." in every directory is set to the "canonical" parent of that directory, despite any other links that might exist.
If you are a bit paranoid (or work with people like I was in years gone by, who like to do things like making hard links to directories and other twisted manipulations of the file system) you might also be concerned about multiple mounts of file systems and directories that are possible on some systems, particularly with some loopback file systems. Usually one would only have one read/write mount at a time, but I understand multiple read/write mounts are possible in some cases.
