Spark: Custom Validator for registration?

In Spark, I'm trying to swap out the default registration Validator for a custom one.

(Why? I need to do some fancy checks on the email address that go beyond the scope of simple validation rules.)

In Laravel\Spark\Interactions\Auth\CreateUser, I found a baseValidator method that defines the Validator for registration. So then I tried setting up a Spark::swap in SparkServiceProvider:

Spark::swap(
    'CreateUser@baseValidator',
    function($request){
      $validator = Validator::make(
          $request->all(),
            [
                'first_name' => 'required|max:255',
                'last_name' => 'required|max:255',
                'email' => 'required|email|max:255|unique:user',
                'password' => 'required|confirmed|min:6',
                'country' => 'required',
                'terms' => 'required|accepted',
              ]
      );

      if ($validator->fails()) {
      
     // I'll insert code to handle special email validation cases here.
         
       return back()
            ->withErrors($validator)
            ->withInput();
      }
    }
);

But it appears that this Spark::swap call is being completely ignored. When I attempt to register, the default validation rules are run instead of the ones defined above.

Does anyone know what I'm doing wrong? Thanks in advance!