a.rb

class Foo
  def initialize
    @hooks = []
    @methods = {}
    @existing_methods = self.methods
  end

  def add(mode=nil, &block)
    @hooks <<  block
  end

  def hola
    p :hola
  end

  def to_proc(methods={})
    hooks = @hooks
    methods.each do |name, meth|
      if @existing_methods.include?(name)
        raise ArgumentError, "conflicts with existing method"
      end

      unless self.respond_to?(name)
        self.define_singleton_method(name) { |*args, &block|
          (@methods[Thread.current] ||= {})[name].call(self, *args, &block)
        }
      end
    end
    this = self
    proc do |*args|
      begin
        methods.each do |name, meth|
          (@methods[Thread.current] ||= {})[name] = meth
        end

        hooks.each do |hook|
          this.instance_eval(*args, &hook)
        end
      ensure
        @methods.delete Thread.current
      end
    end
  end
end

class Bar
  def a
    @a ||= Foo.new
  end

  def baz
    a.to_proc(hello: method(:hello)).call
  end

  def hello(a)
    p :hello
  end
end

bar = Bar.new
a = bar.a

a.add { hello }
a.add { hola }
a.add { p 2 }
a.add { |a| p a }

bar.baz