I have an encrypted disk. How do I customize the Guest shell that is offered when I boot? I have an encrypted disk and when I boot I'm offered the ability to log in as a guest and get a Safari only shell.  This appears to be different from the guest account that is offered in the login screen on most non encrypted macs

I'd like to add additional things to that account that will assist me in recovering the laptop in case it is lost or stolen.

How can I customize this safari only guest account?
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).