Neat idea.

A friend of mine made a similar animated GIF type captcha a few years ago but based on multiple scrolling horizontal bars that would each reveal their portion of the underlying image including letters, and made a (friendly) bet that it should be pretty hard to solve.

Grabbing the entire set of frames and greyscaling them, doing an average over all of them and then applying a few minor fixups like thresholding and contrast adjustment worked easily enough as the letters were reveleaed in more frames than not (I don't think that would affect the difficulty much though if it were any diffierent). After that the rest of the image was pretty amenable to character recognition.