The current link-by-hash functionality requires a DIR argument to specify the directory which would contain the hash hardlinks. I propose adding an option that would automatically determine the directory, to ease automation of this feature. For example (given proper ownership and permissions), the target filesystem's root directory (i.e. "/export/ftp") could be suffixed with a hardcoded value (i.e. +"/" + ".rsync-hash") and the resulting string "/export/ftp/.rsync-hash" would be used automatically for all files pouring into the "/export/ftp" filesystem (as long as the options for the rsync process local to this machine would include the proposed new automation option).
I see this being a script that someone wraps around rsync to automate the adding of the option.