I have an encrypted disk. How do I customize the Guest shell that is offered when I boot?
You can't in a reasonable way. The OS X native guest account is wiped after each use. You may be better off using a separate user account and limiting that using parental-controls.
Keep in mind that this will require that user to be able to decrypt the hard drive with their password. So this can only be of benefit if your laptop is stolen while in sleep mode (where the decryption password is already in RAM).

Your best bet is enabling “Find my Mac” which will activate if the guest connects to the Internet (source).