I'm curious if you ever found an answer. The best that I could do was try to persuade myself that asynchronous job output had a timer on it, the directory doesn't allow listing, and the job GUID would be very difficult to guess (for a human at least). Certainly there's a better answer out there... you spend all this effort setting up token-based security for the services, it seems silly that the output isn't similarly locked down.
(Virtual Directories Security Enabled)—A boolean that indicates if the server's virtual directories are secured and require authentication.
If, accessing the content in the arcgisoutput, arcgisjobs, and arcgisinput directories over HTTP will require user authentication. This will negatively impact performance.