Nice little headscratcher that one! I don't think there would be trust in a third party these days. Technology will in the future have to ensure they would be kept on the straight and narrow and its too much to ask right now.
Many would argue no trust within the family either!

If I were to have to draw up a will now, I think I would ensure a thumb drive or 'little black book' with codes and access to wallets would be handed down, that would be the go.