What is new about our idea is that we do not require any change to any part of the filesystem implementation in the kernel. All that is required are RPC and NFS, making the system much more widely applicable.
Though at first it may appear that amd can do what hlfsd does, it can't. Amd cannot return different pathnames as a value of a symbolic link depending on who accessed it. See also Section 2.3 for more details on various ways in which amd cannot help the way hlfsd can.
Several such programs exist, most notably IMAP and POP. However, use of these servers is limited at this time because most MTAs and UAs have not been converted to use them, or they require special environments (the Andrew Message Delivery System requires AFS). Porting those applications for most popular environments is not going to be an easy task. Nevertheless, the benefits of such services over that of hlfsd would include faster and more reliable service, plus greatly expanded functionality (possibly providing threads information for threaded mail readers).