Perhaps a better method for less trustworthy heirs is to give them half of a worded password. Say the password is "pond elephant evergreen tennis skyscraper electric". You give them "pond elephant evergreen" now (in searchable e-mail), and let the dead man's switch give them "tennis skyscraper electric".
Alternatively, let the dead man's switch alert heirs to swap their half of the password, so that all heirs now have the full password. Then you don't overly depend on the dead man's switch.
That's one of one of the main use cases I had in mind when I wrote this: https://github.com/ryancdotorg/threshcrypt - you encrypt a file using N passwords and require that at least M passwords be presented in order to decrypt. At the moment it's not suitable for non-technical users, though you could make a bootable thumb drive that autoruns it and saves the decrypted file. I was also able to embed it in an initramfs on Debian for use with full disk encryption.
Alternatively, let the dead man's switch alert heirs to swap their half of the password, so that all heirs now have the full password. Then you don't overly depend on the dead man's switch.