This is shit, but not quite as shit as my other attempt last night which is so shittingly shitty I refuse to show you it in motion - there's a photo below that doesn't look quite so bad. It's just really hard to get enough accuracy to be able to animate the mapping smoothly. It kind of worked better with just solid garish colours like this, but still isn't what I was hoping for.
Stop Mo' Mapping Test from Retchy on Vimeo
So this is how I tried it first. It looks ok when it's still, but in motion the edges where the small box emerges jump all over the place. Maybe it was just my impatience with this kind of animation though - I think I'll give it another go tonight and perhaps it'll turn out better now I know what to expect. Although having said that, I'm struggling to see how I'm going to use this technique for anything other than rubbish looking 'tests' in the future.