>cocomyles.com
fwiw the concepts seem quite do-able in php, and maybe through javascript too. I would assume they have a grayscale image as the 'base' of a design, and the colours are just overlays... as are the accessories. the main drawback with php is it'll be generated server side unless you cache all the permutations of design.
I've seen much simpler php classes for engravings and other text manipulation, this image processing just adds a few layers of complexity.
>shoesofprey.com
their images are cached. when you click on a custom product, it fetches 10 or so images so you can rotate the product, it isn't 'on the fly'.