Bindable isn't working on your proxy methods because Proxy isn't an EventDispatcher subclass. Proxies communicate changes to the rest of the system by Notification, not Events. We are trying to separate the model from the view, so rather than binding a view component directly to a Proxy to get updates, we have Mediators which respond to the notifications sent by the proxy and update the view accordingly.
The Proxy recieves updates from the view via mediators listening to the view components events, then sending the changed data in a notification picke d up by a command that modifies the model, or the mediator may do simple updates directly to the model on behalf of the view component.
The process is essentially the same as what happens under the hood with Flex binding, except, being built into the framework, it is portable to places where Flex binding isn't available, such as flash.