Ok. Well, I understand your desire for this, but unfortunately there is very little chance you're ever going to see anything along the lines of a mini ITX sized enclosure that supports an ATX sized power supply, or other standard hardware. I'll keep an eye open but to get what you want I think you're going to have to either be willing to settle for something a little bigger than what you want, or be willing to go with a smaller power supply and motherboard form factor.
I mean, the bottom line is, most cars on the road should be designed so that they can consistently get 60mpg or more without relying on electricity to do it, and we should be able to since we already had cars that could do 50mpg back in the early 90's, but unfortunately MOST people want cars with larger capacities or more horsepower, so manufacturers have little reason to mass produce those more efficient kinds of vehicles at this time. It's much the same in this case, There simply isn't enough demand for something like what you want for manufacturers to spend the money on R&D, tooling, manufacturing, marketing and materials, not to mention assembly and shipping, for a product that very few people are going to want. Most people wanting something this small are perfectly willing to use mini ITX hardware, so that eliminates the majority of potential people who might even care about this kind of configuration.