It's a great idea, I've been working on some similar products but in the end it always comes down to the fact that if you give someone the private key to etch/engrave/print, it's not secure. If you password protect it, it defeats the purpose somewhat of having a backup.
Hard to imagine a way around this before 3d printers take off.
You could write down the password in a way that only you would understand, then laminate that and stuff it in a magazine in your attic. Even if someone finds the password paper (which they won't) they won't understand it anyway. And as if a common street thug who breaks into your house is even going to care about stealing this thing that looks like a paperweight.