helper.js

UI.registerHelper('_', function() {
  arguments = _.toArray(arguments);
  var self = this,
      fn = arguments[0];
  arguments.shift(); // Removes the Underscore function
  arguments.pop();   // Remove the Spacebars appended argument
  return _[fn].apply(self, arguments);
});

/*

Example:

{{#if _ "isString" "bonjour"}}
  It is!
{{else}}
  It is not :(
{{/if}}

Would return "It is!"

*/