I don't understand how you plan to install the cable without a lock. You cannot pinch the cable inside the saddle clamp, if that is what you were thinking. You could pass the cable through one saddle rail then put the cable end loop over the seatpost, then clamp the saddle in the post, but the thief can simply undo the saddle clamp with his allen wrench, which is anyway how he'd take the saddle if you didn't have the cable.

The cable is meant to be used with a lock, presumably the same lock that will fasten your bike to the bike rack.

You can use some old bike chain, antitheft nuts/bolts, or the trick of filling the Allen bolt socket with something (ball bearing, silicone caulk) for varying degrees of security. Or put an anonymous looking cover over the saddle.