base.rb

module AutoloadMethod
  def autoload_method(method_name, filename)
    self.instance_eval <<-SRC
      def #{method_name}(*args, &block)
        require #{filename.inspect}
        #{method_name}(*args, &block)
      end
    SRC
  end
end

module ActiveRecord
  class Base
    extend AutoloadMethod
    autoload_method :xss_terminate, "xss_terminate_method"
  end
end

puts "Environment loaded. Time to run the app..."

class Post < ActiveRecord::Base
  xss_terminate :body
end

output.txt

$ ruby base.rb 
Environment loaded. Time to run the app...
Loading up all of XSS terminate now... (may take awhile...)
Called with: :body

xss_terminate_method.rb

puts "Loading up all of XSS terminate now... (may take awhile...)"

ActiveRecord::Base.class_eval do
  def self.xss_terminate(attr)
    puts "Called with: #{attr.inspect}"
  end
end