Hmm, in the absence of any other more helpful replies thus far, my two bob: Unless this is an out-of-the-box transition type, I can't imagine implementing this any other way than using GL: Copying the view as a texture onto a grid and deforming it appropriately.
You won't get the kind of control you need for a 'suck' animation with a standard CA transform, unfortunately, as far as I know!
Best of luck,
Michael