Thanks to Nicholas Riley for providing the incentive to come up to speed on rsync:

Here's how I set up rsync to work automatically from Radio. The problem was in getting SSH authorization to a process executed with sys.unixShellCommand. Radio just executes commands, it doesn't provide any way of connecting standard input. Standard output is returned as the result of the command, and standard error goes to the console log (open Console.app to see it).

It's now just a small step to get my Radio-controlled static sites weaned off the mirroing FTP set-up I currently use.