Can each frame can generate it's own mask? if each frame is it's own photoshop file the action could be recoreded.. and re-run smoothly enough...  just how to asemble and make it interactive in flash would be beyond my current  skills in that program at the moment.

if all the frames are in one file and all the coresponding alpha masks are in one file, a slightly more elaborate  action could be implimented. it's just a matter of letting the  action know  who's boss by telling it to treat each frame's  mask individually... good luck