You could try the kind of slightly thick paint you paint motives on clothes, like t-shirts with. I've used that on leather gloves and it holds up nicely being slightly "rubbery", but does crack over time. Shouldn't cost much to give it a try. Instructions for my paint say the motive should be "hardened" by ironing on it with some towel or something in between, but I don't think that's necessary, especially since you want it to crack a bit.