I suppose it's possible. It's not like the moka pot is generating a seal on par with a standard pressure cooker, as far as I know- the stats I've seen for it indicate a maximum of 1-2 bars of pressure- so it doesn't need as tight of a seal. A good piece of leather of the right thickness might conceivably do it.

mind you an apocalyptic situation without access to replacement rubber gaskets is probably an apocalyptic situation without access to coffee grinds, so it's probably best to think of this as more of a MacGyver situation. "Oh no! I and my fellow travelers, who wanted espresso on our fourteen day trip into the wilds of the Yukon, have found that our moka pot's gasket broke due to cold and the backup broke due to the dog finding it and using it as a chew toy! How do we make our coffee now?"