When you render to, for instance, photoshop file format, you have to go to 'G-buffer' and tick the box for 'mask', or 'alpha mask' (I have to remember because I am used to C3 now). In photoshop you then have a extra channel, from which you can make layer mask. But, you want an animation. I guess you have to choose 'sequenced tiff' or something, whatever gives you the opportunity to get that G-buffer thing going. If you have those sequenced files (or, if nothing works a lot of seperate renders in which you render each frame in the batch renderer), in photoshop you make a selection of the 'Mask' channel, make a layer mask and save the image as a .png image, which you can use in Flash. I guess more programs can do the last steps. This counts as 'any help', does it?