Command-line arguments are permitted in COMMAND provided that COMMAND is presented to rsync as a single argument. You must use spaces (not tabs or other whitespace) to separate the command and args from each other, and you can use single- and/or double-quotes to preserve spaces in an argument (but not backslashes). Note that doubling a single-quote inside a single-quoted string gives you a single-quote; likewise for double-quotes (though you need to pay attention to which quotes your shell is parsing and which quotes rsync is parsing).