My idea would be more of a proximity sensor that not only does the RFID-walkaway thing, but also locks the machine up if anyone additional to yourself approaches the device.
But then the LEO agent sees this, and not only does he pick up the keys, he will then proceed to charge you with something for resisting arrest, or obstruction of justice or any of the myriad of ways they have to make your life a living hell.
I think this would be similar to smoking pot in a moving car, then getting a police car to tell you to stop and then you throw the drugs out the window. I believe that's enough of a reason to charge you with all sorts of nasty stuff.
I was trying to say that it seems to me as an ineffective method. Just as ineffective as trying to argue with the police that since you don't have the drugs on you (because you just tossed them) you are not in trouble. The drugs don't disappear just as the keys won't, so the agents would just have to get the keys and unlock your laptop.
Of course, just like the shoe bomber changed airport security forever, an rfid keyfob like you suggested becoming commonplace, would just lead to the cops immediately tazing and tackling you, and treating every single motion of your arms as an attempt to destroy evidence.