gistfile1.js

    var MS = function(config) { 

         /* Whatever constructor stuff you wanted. 
            No point chaining, since there's no inheritance, 
            and Y.ModelSync.REST's constructor and initializer 
            will get called by the infrastructure 
         */ 
    };

    ...

    MS.prototype.getURL : function (action, options) {

            if (Lang.isArray(this._shelves)) {
                return this.root;
            }

            // other conditions here
            return Y.ModelSync.REST.prototype.getURL.apply(this, arguments);
    };

    MS.prototype.sync = function() {
          if (someoverridecondition) {
               // new logic
          } else {
               Y.ModelSync.REST.prototype.sync.apply(this, arguments);
          }
    };

    Y.GalleryModelSync = MS;