I was thinking that if secure timestamping could be done quickly enough, it might be possible to both bound the event by the future-light-cones of the spacetim

I was thinking that if secure timestamping could be done quickly enough, it might be possible to both bound the event by the future-light-cones of the spacetime points the GPS satellites sent the signed signal, and the past-light-cones of the timestamping event, but I suppose that’s probably not feasible (even with signed GPS signals). Now, maybe on an interplanetary scale (with various sources playing the role of the GPS satellites, and various locations doing secure timestamping) it could be enough in order to be able to establish (beyond plausible hope of spoofing) a kind of rough region in space-time, but, maybe not very precise.