How about using straight forward function callbacks?
When you create your PopUpPanel, pass in a function callback that you want to be executed when the popup is closed. Rather than using an event, you'd simply call the function. i.e.
Inside FormPM:
public function showPopup():void
{
var popup:PopUpPanel = new PopUpPanel();
popup.onCompletion = handleResult;
PopUpManager.addPopUp(popup, ...);
}
private function handleResult(someData:*):void
{
// My popup has completed.. what do I want to do with the result.
}
You might want to consider the Spicelib 3.0 command framework and have a command to popup your panel and then add the error/success callbacks to that: http://www.spicefactory.org/parsley/docs/3.0/manual/?page=commands§ion=intro