You could try conditional validation: http://batmanjs.org/docs/api/batman.model_validations.html#conditional_validation
@validate "name", presence: true, if: "id"
where presence of "id" shows that it has already been created. You could define another accessor and use that instead of "id" if you need a more robust test.
In the long run, it would be nice to imitate Rails' `validates ... on: ` in batman.js!