If your purpose is to secure your bean property, i think you should create an intermediate bean for your User creation. Tapestry Form properties are mapped to bean properties, and properties are set simply via accessors. Tapestry requires an empty constructor to your bean, and this is how Tapestry instantiate an object for the form.
I might be wrong, but i think that if you do not want to modify your user 'model' (and add an setter) then you should use a specific bean for form interaction and create the complete User Entity (using constructor) in the success phase.
