Is it possible to define a Ruby singleton method using a block?

def define_say(obj, msg)
  def obj.say
    puts msg
  end
end

o = Object.new
define_say o, "hello world!"
o.say

def define_say(obj, msg)
  obj.define_singleton_method(:say) {
    puts msg
  }
end

def define_say(obj, msg)
  # Get a handle to the singleton class of obj
  metaclass = class << obj; self; end

  # add the method using define_method instead of def x.say so we can use a closure
  metaclass.send :define_method, :say do
    puts msg
  end
end

>> s = "my string"
=> "my string"
>> define_say(s, "I am S")
=> #<Proc:0xb6ed55b0@(irb):11>
>> s.say
I am S
=> nil