Isn't the address created from a passphrase based on a defined algorithm? can't you check what address will be created for a specific passphrase offline then?
The account number is generated from the public key, not the pass phrase.
Here is a more detailed explanation from Boris:
your passphrase -> make hash from it -> make public and private key from it -> from public key you can make address
hobogo, Boris will give you a response later on.

He is busy with 0.1.9 right now.