Perhaps instead of PGP you could divide the key into (2?) parts, sending the mail to 2 different people. People are more honest when there's another person there. Send these mails to a second group in case one of the original pair loses their email (i.e they were one of those idiots who used a non portable email from their ISP). Alternatively perhaps it could be written into a Will and sent out by mail on death.

Also, do we know of more reliable email scheduling? This thing might have to survive for hundreds of years.

I think it's got to be very useful for protection from extortion too if it can be designed so even us ourselves don't have access. For example putting half of the key out of reach to us somehow?