What would really be effective in all three of these cases is that, because the girls obviously like to show themselves off, just being around them would be a torment for the chastity slave. And if they decided it would be funny to give him extra teasing, visual or otherwise, his frustration and discomfort would just get worse and worse.
They're all quite different, though. The last scenario is one I have done quite a bit: the cultural fish-out-of-water idea. I love the idea of a girl mentally saying "well, I don't know how the fuck this happened, but free orgasms, hell yeah!"