Interesting concept honestly, but my question comes down to the passphrase. How would they know How would the private key be encoded with a second passphrase and only they (the customer) would know it?
Hi. We recommend to use this service
https://walletgenerator.net to generate Crypto address and pass-phrase protected private key. Please try to generate paper wallet and hit checkbox "BIP38 Encrypt?". After that you need to enter passphrase that will be needed to encode your key. And nobody except you will know that passphrase.