It seems to me that if the concept of "parallel regions" is to be interesting, we should not narrow our focus to "hardiness", nor expand our focus to "growability". Obviously hardiness is only one dimension of a plant's requirements; when we search for parallel regions, we should also consider whether a place gets winter rains or summer rains; whether the summmer temperatures are high or low; whether there is a thick snow or not during the winter; whether the climate is seasonal or constant; whether the climate is humid or dry; whether there is dew or not during the driest time; etc. If two areas share these climate characteristics, we can be confident that plants from those areas could be swapped rather easily. We might want to consider soil characteristics as well, but since that is something all gardeners control anyway, maybe that would be cutting things too finely to be useful.
Lou