And so I abandoned the Ruby to Node port after writing this:

or.js

Function.prototype.or = function(callback) {
  var next = this;
  return function(err, doc) {
    if (err) {
      callback(err, doc);
    } else {
      next(doc, callback);
    }
  };
};

Thanks Till! I'd come across task.js, but not Spidernode. Will bookmark for future reference.