If so: would it be useful for anything? would it be risky as BTC and NMC are linked.
Yes, but risk will unquestionably be higher.
You could encrypt it with a hashed passphrase of about two hundred length for English, keep the hash to yourself, and unlock it by hashing your passphrase whenever you need it to unlock the encrypted key.
This is redundant for coins that allow one to hash passphrases into keys. That would be the preferred method: use a passhrase to key system then the private key is never stored anywhere except in memory, yours and the logged client.
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.