Need to animated subtracted rounded corner shape

Hello, 

I need to produce a png animation by subtracting animated rounded corners rectangles from another shape. I managed to simulate what I have in mind in the attached gif. Of course I used white as a background to achieve this effect but I need true transparency. I am really trying to achieve is something like the subtract from illustrator pathfinder. Unfortunately I need to covert everything to symbols to keep 9 slice scaling (is this correct, and if I do so combine objects doesn't work (do I understand right, no 9 slice scaling outside symbols?) and by doing so I also lose the transparency around the corners. 

 Whas wondering if I could just import intermediate frames from illustrator and tween between them. Not really sure how to do that, so if you could point me to some resource I'd really appreciate it. 

Thank you very much