a.rb

module Mixin
  Code = proc do
    a_class_level_method(self.name)
  end

  def Mixin.included(other)
    super
  ensure
    other.module_eval(&Code)
  end
end

class C
  class << self
    attr_accessor :it

    def a_class_level_method(value)
      @it = value
    end
  end
end

class B < C
end

C.send(:include, Mixin)
B.send(:include, Mixin)

p C.it
p B.it

__END__

"C"
"B"