Can I bind a Flex component property to a function?

private function isUserAllowed (userName:Boolean):Boolean {
   if (userName == 'Tom')
      return true;
   if (userName == 'Bill')
      return false;
}

<mx:Button label="Create PO" id="createPOButton"
enabled="<I want to call isUserAllowed ('Bill') or isUserAllowed ('Tom') here>"
click="createPOButton_Click()" />

<mx:Button
enabled = "{this.myFunction(this.myVariable)}"
>

<mx:Button
enabled = "{(function(arg:Object):Boolean{ ... })(this.myVariable)}"
>

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">

    <mx:Script>
    	<![CDATA[

    		[Bindable]
    		private var currentUser:String = "Bill";

    		private function isUserAllowed(user:String):Boolean
    		{
    			if (user == "Bill")
    			{
    				return true;
    			}

    			return false;
    		}

    	]]>
    </mx:Script>

    <mx:VBox>
    	<mx:Button label="My Button" enabled="{isUserAllowed(currentUser)}" />
    	<mx:HBox>
    		<mx:Button label="Try Tom" click="{currentUser = 'Tom'}" />
    		<mx:Button label="Try Bill" click="{currentUser = 'Bill'}" />
    	</mx:HBox>
    </mx:VBox>

</mx:Application>

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" initialize="init()">

    <mx:Script>
    	<![CDATA[

    		private var _currentUser:String = "Bill";

    		public function set currentUser(value:String):void
    		{
    			if (_currentUser != value)
    			{
    				_currentUser = value;
    				dispatchEvent(new Event("userChanged"));
    			}
    		}

    		[Bindable(event="userChanged")]
    		private function isUserEnabled():Boolean
    		{
    			if (_currentUser == "Bill")
    			{
    				return true;
    			}

    			return false;
    		}

    	]]>
    </mx:Script>

    <mx:VBox>
    	<mx:Button label="My Button" enabled="{isUserEnabled()}" />
    	<mx:HBox>
    		<mx:Button label="Try Tom" click="{currentUser = 'Tom'}" />
    		<mx:Button label="Try Bill" click="{currentUser = 'Bill'}" />
    	</mx:HBox>
    </mx:VBox>

</mx:Application>

<mx:Script>
<![CDATA[

    [Bindable] var _username : String;

    private function isUserAllowed (userName:Boolean):Boolean {
        if (userName == 'Tom')
            return true;
        if (userName == 'Bill')
            return false;
    }

]]>
</mx:Script>

<mx:Button label="Create PO"
    id="createPOButton"
    enabled="{isUserAllowed(_username)}"
    click="createPOButton_Click()" />

<mx:Script>
    <![CDATA[
    	[Bindable]
        private var userName : String = "Tom"; // or whatever ...
    	[Bindable]
        private var userAllowed : Boolean;

        private function isUserAllowed ():Boolean {
           if (userName == 'Tom')
             return false;
           if (userName == 'Bill')
              return false;
           return false;
        }
    ]]>
</mx:Script>

    <mx:Button label="Create PO" id="createPOButton" enabled="{userAllowed}" addedToStage="isUserAllowed()"/>