Rails4.0.0でJoinしたテーブルに対してreadonly(false)が効かないのはバグです。4.1系で解消されるはずなので、次のパッチをconfig/initializersにでも入れておくといいよ。

relation_patch.rb

# -*- coding: utf-8 -*-

module ActiveRecord
  class Relation
    def initialize(klass, table, values = {})
      @klass  = klass
      @table  = table
      @values = values
      @loaded = false
    end

    private
    def exec_queries
      @records = eager_loading? ? find_with_associations : @klass.find_by_sql(arel, bind_values)

      preload = preload_values
      preload +=  includes_values unless eager_loading?
      preload.each do |associations|
        ActiveRecord::Associations::Preloader.new(@records, associations).run
      end

      @records.each { |record| record.readonly! } if readonly_value

      @loaded = true
      @records
    end
  end
end