gistfile1.rbx

module M
  def subclasses
    @subclasses ||= []
  end

  def inherited(subclass)
    subclasses << subclass
  end

  def const_missing(name)
    subclasses.each do |subclass|
      return subclass.const_get(name) if subclass.const_defined?(name)
    end
    super
  end
end

class A
  extend M

  def x
    C
  end
end

class B < A
  C = 100
end

p B.new.x  # => 100
p A.new.x  # => 100