If I pass a file as --link-dest, rsync complains "--link-dest arg is not a dir". But this makes perfect sense and would be useful:
rsync -a --link-dest=prevfile srcfile dstfile
See also http://stackoverflow.com/questions/7100900/rsync-link-dest-option-for-single-file
If you have just one file you can run the ln yourself. Rsync needs the file in a dir using the same name for the link-dest option to work.