Nice idea , but whats the use of it when it comes to whatsapp uses your phone number ? they will identify you by that one thing !
Yeah, as said you can choose any password during the registration. The Problem just is that if you are using a password that is different from the default passwords you may can get caught for using a "non-official Program". I don't know how whatsapp saves the passwords but if they save them in plain-text (what they shouldn't do) they can delete/block your account even when you already did register. :