Greetings,
While I do think it a good idea, there might be a problem or two with it. Many of the places that folks are living out of their cars currently are not places they could put up any structure, especially not one that is as seldom seen as a hexayurt. The US has a tendency to actually put architectural features in place to keep the visibility of homeless folks to a minimum in the first place, this would have the opposite effect and would probably end up making the possibility of them even being able to stay in their cars that much more difficult, unfortunately. Also, the "storage on a roof rack" becomes difficult if the car is actually in use, because that is a large light package, and unless you have a really good way to not only secure it, but keep it from dropping your miles per gallon even more significantly, it impedes the use of the car as a vehicle as well. There is also the funding for such that becomes a question, while it's not all that expensive on the scale of, say, buying a house, when one is worried if one's children are going to be able to get the next meal, or worried about the gas prices, the cost of a hexayurt might be as far away as the cost of a house or apartment. Funding more family-friendly shelters might be a more short-term and long-term productive solution, unfortunately.
Just some thoughts