t.rb

class C
  # I want to define methods on this
  # particular C object that access
  # stuff from b.
  def self.from_b b
    self.new.tap do |c|
      eval "def c.foo; #{ b.foo }; end "
      eval "def c.gob; #{ b.foo }; end "
    end
  end
end

class B
  attr_accessor :foo
  def c
    C.from_b(self)
  end
end

require 'test/unit'

class T < Test::Unit::TestCase
  def test_this
    b = B.new
    b.foo = 2
    assert_equal b.foo, b.c.foo
    assert_equal b.foo, b.c.gob
    b.foo = 3
    assert_equal b.foo, b.c.foo
    assert_equal b.foo, b.c.gob

    assert_raises(NoMethodError) { C.new.foo }
  end
end