There are two points of failure here :
1 ) the id of the SWF object element is currently set to ‘fancybox-swf’ which contains one of the proscribed characters according to Adobe documentation : http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/external/ExternalInterface.html
excerpt : "Note: When embedding SWF files within an HTML page, make sure that the id attribute is set and the id and name attributes of the object and embed tags do not include the following characters: . – + * / \"
I see three levels of solution :
1 ) Fix the ‘-‘ in the id of object and add a different id to embed in order to facilitate a ‘roll-your-own’ solution
2 ) Add an API that returns a reference to the Flash Player by function name : like getFlashPlayerForFunctionCall( functionName )
3 ) Full boat solution ‘callActionScriptFunction( functionName, paramArray ) and then use ‘apply’ to call the function and passthrough/return the result
I documented my workaround on Stack Overflow : http://stackoverflow.com/questions/22875712/externalinterface-callback-not-working-in-fancybox