I had just an idea about your problem, this is of course a fake
solution, but maybe it is helpful.
If you can live with the warning or if you can avoid to use constants
for classes you could always save a copy of the class before including
the module and than "uninclude" the module by restoring it, of course
this means  that all other potential operations on the original class
will be lost too :(

module Outer
  module N
    def a; 222 end
  end
  module M
    def a; 42 end
    def b; 101010 end
  end

  class C
    include N
  end
  copie = C.clone
  class C
    include M
  end

  puts C.new.a
  puts C.new.b
  const_set "C", copie
  puts C.new.a
  puts C.new.b
end

Cheers
Robert
-- 
http://ruby-smalltalk.blogspot.com/

---
Whereof one cannot speak, thereof one must be silent.
Ludwig Wittgenstein