Change to Backbone.Validations to allow for a "onlyWhen" property that runs a model function, and if it returns false the validator is skipped.

backbone.validation.onlywhen.js

var getValidators = function(model, attr) {
        var attrValidationSet = model.validation ? model.validation[attr] || {} : {};
  
        // If the validator is a function or a string, wrap it in a function validator
        if (_.isFunction(attrValidationSet) || _.isString(attrValidationSet)) {
          attrValidationSet = {
            fn: attrValidationSet
          };
        }
  
        // Stick the validator object into an array
        if(!_.isArray(attrValidationSet)) {
          attrValidationSet = [attrValidationSet];
        }
  
        // Reduces the array of validators into a new array with objects
        // with a validation method to call, the value to validate against
        // and the specified error message, if any
        return _.reduce(attrValidationSet, function(memo, attrValidation) {
          // Don't run this validator if the onlyWhen function returns false.
          if (attrValidation.hasOwnProperty('onlyWhen')) {
            if (!model[attrValidation.onlyWhen]()) {
              return;
            } else {
              delete attrValidation.onlyWhen;
            }
          }
          _.each(_.without(_.keys(attrValidation), 'msg'), function(validator) {
            memo.push({
              fn: defaultValidators[validator],
              val: attrValidation[validator],
              msg: attrValidation.msg
            });
          });
          return memo;
        }, []);
      };