gistfile1.js

mainApp.directive('unique', function($http) {
  return {
    restrict: 'A',
    require: 'ngModel',

    scope: {
      ownId: '@uniqueOwnId'
    },

    link: function(scope, elem, attr, ctrl) {
      scope.$watch(attr.ngModel, function(value) {
        console.log(value);
      });
    }
  }
});

gistfile2.html

<input type="text" ng-model="customer_tag.name" unique unique-own-id="{{customer_tag.id}}" />

En faisant ça, ng-model n'est plus évalué car son contexte a changé.