I thought they were already quite busy. If they're so busy that you don't feel you can attach a local tape drive, I'm not really understanding how they're unbusy enough to hammer them with a bunch of rsyncs.
Another point: Is buying and attaching all that disk to the SUN systems really cheaper than simply attaching a local tape drive? And setting up a cluster with SnapMirror would probably be more expensive, but not prohibitively so.
Bruce