There must be an historical case where someone had to prove that they are not dead, no ?
But in those cases, you get back to the issue of proving identity, since if you've been in a situation where you're presumed dead, you might not have anything left but your body. Also, such situations are rare enough that they'd be dwarfed by more mundane cases of others trying to claim the identities of the dead, so there would be net pressure to make proof-of-liveness more stringent.