Edit in JSFiddle

var Test = new Class({
    Implements:[Options],
    'private initialize':function(options, _private){
        this.setOptions(options);
        _private.test = 'Hello, '+ (options.name || 'world!');
    },
    'private say':function(i_can_call_this_as_i_want){
        alert(i_can_call_this_as_i_want.test);
    },
    a_normal_method:function(){
        alert("I don't use privates");
    }
});

    var kenta = new Test({name:'kenta'});
    kenta.say();
    kenta.a_normal_method();

    var test2 = new Test({});
    test2.say();

    //when done
    test2['~']();
    kenta['~']();

<script type="text/javascript">
/* 
---

name: Class.PatternMutators

description: Pattern-Based Mutation for MooTools 1.3

license: MIT-style license.

copyright: Mark Obcena

requires: Class

provides: [Class.defineMutator, Class.define]

...
*/


(function(){

var matchers = [];

var lookup = function(key){
    var i = matchers.length;
    while (i--){
        var matcher = matchers[i],
            match = key.match(matcher);
        if (match) return ['$mutator:' + matcher, match.slice(1)];
    }
    return null;
};

Class.defineMutator = function(key, fn){
    if (typeOf(key) == 'regexp'){
        matchers.push(key);
        key = '$mutator:' + key;
        var _fn = fn;
        fn = function(values){
            return _fn.apply(this, values);
        };
    }
    Class.Mutators[key] = fn;
    return this;
};

var define = Class.prototype.implement;
Class.implement('define', define);

var implement = Class.prototype.implement = function(key, value, retain){
    var mutator = lookup(key);
    if (mutator){
        key = mutator.shift();
        mutator[0].unshift(value);
        value = mutator.shift();
    }
    return define.call(this, key, value, retain);
}.overloadSetter();

// Default Mutators
Class.defineMutator(/^protected\s(\w+)/, function(fn, name){
    this.define(name, fn.protect());
});

Class.defineMutator(/^linked\s(\w+)/, function(value, name){
    this.prototype[name] = value;
});

Class.defineMutator(/^static\s(\w+)/, function(fn, name){
    this.extend(name, fn);
});

// Reimplement "implement" in all classes..
for (var i in window){
    try {
        var klass = window[i];
        if (klass instanceof Function && typeOf(klass) == 'class'){
            klass.implement = implement;
            klass.define = define;
        }
    } catch(e){}
}

})();

</script>
<script type="text/javascript">
/* 
---

name: kenta.Private

description: Private mootator for MooTools 1.3.x

license: MIT-style license.

copyright: Carlesso Cristian http://mykenta.blogspot.com

requires: Class.PatternMutators

provides: private pattern mutator, '~' cleanup method.

...
*/

(function(){
    var Private={};
    Class.defineMutator(/^private\s(.*)/,function(fn, name){
        
        var getPrivate = function(bind){
            var uid= $uid(bind);
            return Private[uid] || (Private[uid] = {});
        };
        
        this.define('~', function() {
           var uid = $uid(this);
           delete Private[uid];
        });
        
        this.define(name, function(){
             var priv = getPrivate(this);
             var args = Array.from(arguments);
             args.push(priv);
             var res = fn.apply(this, args);
             return res;
        });
    });
})();
</script>