How can I rewrite attr_readonly?

Here you should use attr_protectedinstead attr_readonly. Then you will be protected from mass assignment of forms.

#in model
attr_protected :my_field
#in controller
obj = MyModel.new({:my_field => "dsadsad"})
obj.my_field
#=> nil
obj.my_field = "ololo"
obj.my_field
#=> "ololo"

EDIT

Situation: you need to install emailonly once: when creating the user. Then you want to edit the email only if you are an administrator.

# Model
attr_protected :email

# Controller
def create
  @user = User.new params[:user]
  @user.email = params[:user][:email]
  @user.save
  respond_with @user
end

def update
  @user = User.find params[:id]
  @user.email = params[:user][:email] if curent_user.admin?
  @user.update_attributes params[:user]
  respond_with @user
end

Also check out: http://railscasts.com/episodes/237-dynamic-attr-accessible