What's wrong with something like this:
validate :make_one_particular_and_unique_thingy_go_red, :if => :validate_the_thingy
in the controller:
@some_model = SomeModel.some_factory_method(params)
@some_model.validate_the_thingy = true
In both cases, the validation method is sitting around on the model.
I'd argue that this "breaks MVC" *less* than the solution in your Gist, since this keeps the flag separate from its implementation, rather than having the controller poke extra validations (by method name) directly into the model instance.
If you've *really* got bunches of wildly divergent validation setups, I'd agree with the suggestion on the ticket to actually create a (plain Ruby) mediator object and use the ActiveModel APIs.