Seeing the `is_prepended` bit twice and the search through `object_singleton_class.ancestors` here makes me think you could extract a common helper method:

``` ruby
def definition_host
  @definition_host ||= if is_prepended?
    mod = Module.new
    object_singleton_class.__send__ :prepend, mod
    mod
  else
    object_singleton_class
  end
end
```

Then the logic above can just be:

``` ruby
define_proxy_method_on definition_host
```

...and here it can just be:

``` ruby
definition_host.__send__(:remove_method, @method_name)
```