Is it true that if you die while in the service, the military will return your dog tags to your loved ones? Perhaps you could get a private key engraved onto a dog tag and carry it with you, and let your family know how to use it in the event of your death. Does the military even do dog tags anymore?
There are dog tag creators at some Walmarts and pet stores, can also order them online. Though putting a private key in the clear would be asking for it to be swept if archives are kept of what was put on the dog tags. If you order them one at a time from separate sources instead of as a pair at once, you can split the key.
You could also split the key on tats. One tat by one artist just above your genitalia, tats by another artist on your buttcheeks, so each one doesn't see the others' work. Just gotta trust your sexual partners, then.