Not really. If a husband is long gone, a wife is not especially likely to sign for him. She is especially likely to do so while he is alive, as she is signing for him. As, if him is gone why is him signing anything?
Ultimately it is not relevant to the current state of affairs if Biden is dead or alive or replaced with a clone or not-really-president. On its face, he's POTUS and we call him Biden. Or Bidan. A rose is a rose is a rose.