Just posting it here because when I forget how I did it (and I will) I'll look here - and it might help someone.
Import an Alpha channeled FLV saved as an SWF.
For example I have chroma key video between graphics in Flash. The secret is the Animation codec.
It's a bit tricky so I have all the Mac steps here: http://mmtipsntrix.blogspot.com
Retrieving data ...