Most of the productivity of the permaculture garden is not at the edge where it meets zone five, at least that is my perception, I could have some misunderstanding about permaculture.
Well, at least no more than most permaculture instructors. This gets into one of the secrets to permaculture's success that doesn't get much play amongst its practitioners, but all of those insects, animals, and bacteria that make zones 1-4 thrive rely on a healthy zone 5 for their own well-being. You can also take a look at the forests adjacent to Joel Salatin's Polyface Farm as a similar example. Without zone 5, the things that make permaculture work all go away.
As another option, bio-intensive (http://www.growbiointensive.org/) doesn't focus at all on edge and can improve both the soil and yield.
Jeavons' data on yield, well, let's just charitably say that it raises some suspicions. See Toby Hemenway's Gaia's Garden for more discussion. Personally, I think of "biointensive" as, if not a total farce, then at least mostly a farce.
My idea is that the gardens would actually be most extensively spaced out around, on top of, and inside the houses, not the city, though some might be necessarily spaced around the settlement.
Then you would have far, far less acreage, and far less yield per acre, than even the Mayan "garden cities" or modern Havana. Relying solely on what you can put inside a city's limits, even a very spaced-out city, you'll never get enough food to feed any kind of significant population. You'll definitely have to start spacing out fields radiating out from the city in all directions, as new farmers have to walk out to their fields. Which brings you right back to the calculations we talked about earlier. You end up with a population of a few hundred, because you can't feed anything more than that.
This allows you to cover a much larger area...
No, that covers a smaller area. A much smaller area.
ut can only work becuase of taking advantage of modern buildings (mostly - it is possible to do this with smaller structures, but harder, which might be another reason why know one has done it yet). Am I completely off my rocker?
How does a modern building increase the amount of area you can garden? You can do some trivial amount of gardening through windows, but only what you can crowd around the outer wall, and because you have to isolate these plants from a larger ecology to do this, you reduce their yield. On the rooftop, you should have a chance to replace the area you lost by having the building in the first place, but that misses the point. How much soil do you have on the roof? If you have enough to replace the soil that the building sits on, then the roof will collapse. If not, you have plants growing in very little soil, so they can't grow as well or as big. Rooftop gardens certainly make a marked improvement from bare rooftops, but they seem downright pitiful compared to the same area of actual soil.