direct-prepend.rb

class Foo
  def test
    puts "test"
  end
end

class Foo
  prepend Module.new {
    def test(*)
      puts "before"
      super
    end
  }
end

meta-with-prepend.rb

module Mst
  def before(name, &advice)
    mod = Module.new do
      define_method(name) do |*args, &block|
        advice.call
        super(*args, &block)
      end
    end

    prepend mod
  end
end

class Foo
  extend Mst

  def test
    puts "test"
  end

  before :test do
    puts "before"
  end
end