In your case, $scope.myForm.username.$setViewValue($scope.myForm.username.$viewValue); does the trick – it makes both the form and the field dirty, and appends appropriate CSS classes.
Just to be honest, I found this solution in new post in the topic from the link from your question. It worked perfectly for me, so I am putting this here as a standalone answer to make it easier to be found.
EDIT:
Above solution works best for Angular version up to 1.3.3. Starting with 1.3.4 you should use newly exposed API method $setDirty() from ngModel.NgModelController.