I'm already using ActionScript 3. I found out that after removing my script (for tweening image), I can load the swf just fine.
But even when it loaded, the image is all scretched out. Looks like there's problem with GameGo scaling mechanism for swf. Eventhough I'm masking the content (stage.mask is not available I'm afraid) inside a MovieClip, it always detect overall size of the image inside the swf.

My feedback:

1. Is there any restriction when using script to tweening the image dynamically? (I'm using TweenMax from Greenshock in this instance). It seems the app always Force Close when I try to use it. I have tested that Flash native Tween class is ok to use, but is not powerful enough for my needs.
2. I think you need to remove scaling check for loaded swf (or at least masking them within GameGo individually). I don't think there are any method to completely clip the swf content based on it's stage size. If it's loaded in other SWF, it will always detect overall asset size inside the swf.

Ok, I have workaround with scaling problem by adding the animation MovieClip after 1 frame delay. It seems the GameGo will detect the swf size correctly if there's nothing on the stage at the beginning.

I also sticking on using the Timeline to manually tween my banner. Not the most preferred way for me as programmer, but it do the trick :

The new (HTML5) version of GameGO will have a lot more customization options, next to changing colors in the config.ini file you'd be able to edit the HTML/CSS files for really the ultimate customization you'd get.