> it's not guaranteed that there exists a directory on the same filesystem where you have permission to create a temporary file.
this sounds like a red hearing to me.
If you aren't allowed to create a new file in the directory of the file, are you sure you have permission to overwrite the file you are trying to modify?
as for the symlink/hardlink 'issue', in my sysadmin experience, more problems are caused by editors that modify files in place (not using temp files and renaming them) than by breaking links. Editors that break links when modifying a file are referred to as 'well behaved' in this area.