god_mode.rb

def god_mode
  Object.class_eval do
    def method_missing(m, *args, &block)
      if private_methods.include?(m)
        send(m, *args, &block)
      else
        super
      end
    end
  end
end

def mortal_mode
  Object.class_eval do
    def method_missing(*)
      super
    end
  end
end

class Foo
  def bar
    "baz"
  end

  private

  def baz
    "SECRET"
  end
end

Foo.new.bar # => "baz"
Foo.new.baz rescue "ERROR" # => "ERROR"

god_mode

Foo.new.bar # => "baz"
Foo.new.baz # => "SECRET"

mortal_mode

Foo.new.bar # => "baz"
Foo.new.baz rescue "ERROR" # => "ERROR"