I think it seems like a risky idea - you are putting something that others may see as garbage into something that everyone sees as valuable.
A thief would certainly steal a diamond, but probably not a broken plastic toy or whatnot.
If I wanted to have a key engraved, it would rather be into some regular (non corrosive)metal, plastic or glass that will be conceived as a plain object to anyone that is not in the know.
How about a fork with a tooth missing in a "man-drawer" with a key engraved - IMHO that would be much more safe.
While there are definitely markets for that, the market we are thinking about is an ultra-luxury niche. People who enjoy having things like diamonds and things like bitcoin. While the 'hidden in plain sight' is a good idea, it works with diamonds- wealthy people have quite a lot of diamonds around in general, women in particular. Not to mention, it might be something you enjoy keeping in a safe. After all, the engraving is microscopic- the Diamond does not lose any value by being engraved.
Also, a Diamond is a heck of a lot more durable then a broken plastic toy or really anything else for that matter. With the exception of being shattered (and lasers) they are impervious to pretty much everything else.
Besides that- the cool factor says it all. Who wants a billion dollars stored on a broken plastic toy? :-)