Oh hey I just thought of something...
my laser machine can cut AND engrave acrylic. I could make transparent litecoins no problem. Now, no way to hide the private key... but neato as a collector's item.
Perhaps you could mess around with refraction of light to make the private key latent until the coin is cracked or opened, each of which would change visibility.
Or maybe you could make it like a glowstick (dye would go from clear-black), just bend the coin and the ink/dye comprising the private key would appear. I'm not sure if you would need to put the dye on some medium to keep it from being damaged though... The coin would still be mostly transparent.
Perhaps make and outer layer that's UV-blocking and a UV-sensitive private key on and inner layer. That way, it is transparent to the visible spectrum, but once you remove the UV-blocking layer, you can place the private key layer under a black light to view it.