Could I store a bitcoin private key value in a Name Coin? If so:
 would it be useful for anything?
 would it be risky as BTC and NMC are linked.
Yes. So that the whole world can see it? Yes seems like a good idea.

The NameCoin block-chain is public, although you could encrypt it in the NameCoin chain, but why there? If you are scared to lose any privates keys you should work on your back-upping strategies, not uploading it to the public domain in the hope none will realize it is a ECDSA Bitcoin private key.